What can I get to notify me when my grandpa leaves the bedroom, but not when my grandma leaves?

At age 98 I put a small, delicate wind chime on my auntie's walker. You can tape or tie it on. (example ) Amazon / Carson Home Accents CHA63156 Pewterworks Angel Crystal Wind Chime. The earlier suggestion of bells is a good idea also. The second smaller walker I bought is a compact foldable metal walker with 2 wheels. No throw rugs on the floor. We now live in a condo 2bdrm ( 934 sq. ft interior ) and I can hear her when she starts to get out of bed and move around because of the twinkeling of the chimes. The baby monitor is a good idea also. The commode is good for urinating, but my aunt even now ( age 101) insists on making her way to the bathroom ( 18 steps each way) if she needs to have a bowel movement, despite my urging her to use the commode for everything. When my mother was alive ( Alzheimers ) and we lived in a house, we attached a 'plastic' baby gate in the door way halfway up at a height where she could rest her elbows on it and call out ( she was unable to open the gate herself. )This was great because a closed bedroom door kept me from hearing her and also she was not cut off from the rest of us but was kept in her room so she could not wander off. I hope something of this helps in some way or gives you some ideas. This is not an easy journey and each aging adult has their own needs and ways.

Bells? On his ankles or around his wrist or neck?

You can get a motion detection device and put it on his side of the bed, beyond his portable toilet. Amazon has many choices.

Absolutely get a floor alarm on your grandpa side of the bed. Also be very careful sleeping outside there door. Being a caregiver is exhausting I know from experience and sometimes you may not hear them get up they may fall on you and hurt them self and you.
Also absolutely talk to Doctor about low blood sugar ASAP.
Good -luck don’t forget to take care of yourself too!

SocialWorker23: Get a floor alarm for the elder. However, the reason for his low blood pressure and resulting falls MUST be addressed by his physician.

Put tracking collars on them

See if they are willing to sleep on different mattresses. 2 twin beds placed next to each other is the same size as a king bed. You can get a pressure-sensor on Grandpa's side of the bed that will alarm whenever he gets up during the night. Since he is getting up a lot at night, please get him an appointment with his medical doctor or urologist (if he is already seeing one). He probably has an enlarged prostate. There are medications that can help with this problem.

This is unsustainable for you. You can’t be a 27/7 caretaker and sleep on the floor to prevent him from leaving. You need 8 hours of quality uninterrupted sleep before you get sick.

You are not taking care of yourself.

If I understand this correctly, no alarm is needed if he gets up and uses the commode. An alarm is needed if he LEAVES the room. I suggest what they call a hotel room alarm. You can find them on Amazon.

There are floor alarms that will go off if he stands on it. If he is sleeping on the left side of the bed you place the floor alarm there and if he gets out of bed it will go off, if grandma gets out of bed from the right side of the bed it will not alert.
I suppose the "problem" would be is the alarm will wake everyone up.

If he uses a bedside commode is there a reason he leaves the bedroom?

There are alarm systems that are keyed to a doorway and the person wears a bracelet, anklet or lanyard with the device that sets off the alarm. My mother had this in her memory care unit. Here's an article about it https://www.verywellhealth.com/safety-in-dementia-door-alarms-98172.

If they regularly sleep on the same side of the bed and the bed is fairly wide (e.g., queen), then you could put an alert device under his side of the bed that would sound when he gets up. If you want the alert to sound only when he moves away from the bed, but not when he uses the bedside commode, you might be able to set up a motion detector in his movement path if he usually moves along the floor in the same path. However, even if you were able to do this, it might be too late to prevent a possible fall. In either case, if you need the alarm to go off in the room where he is sleeping (because you are there also), then the alarm may wake up your grandmother, too. Possib,y if she doesn't have severe dementia, she would be willing to wear earplugs at night as I do if I am traveling and can't avoid highway noise. They're never perfect, but they might reduce the noise enough that the alarm would not wake her.

There are fall alert systems (necklaces, bracelets and smart phones) but they only work if the person keeps them on and they cost money.

Some people use a video baby monitor (but that only works if the sound of his moving about will wake you) or there are motion-detection systems you can have installed.

Have you thought about putting a commode in his room? Many people opt for this rather than risking falls for their LOs.

He's had an extensive workup and no solution has been found. He takes multiple medications but they have yet to get him regulated. He has Parkinson's and they believe that's simply the cause. When he lays flat his BP is way too high.

What medical workup has been done for this severe hypotension?
It sounds primarily orthostatic, that is change in position effects it.
What medications is your grandfather on that effect his blood pressure?
How often do you measure his blood pressure?
What has the MD said about the severity of this hypotension?

Often it is balance more than anything taking elders down; that often cannot be fixed. But hyptension often can be fixed.
That I know of no such device as you mention exists unless grandfather is willing actually to push a button on a device to alert you.