Why are you worring about HR if you have a PM? By the time HR catches up your well into effort.

I do 4x4 min 6x 3 at 115% of FTP power to improve vo2 max. Def longer than 2 minutes and without worrying about HR.

Btw, 90% is way too easy for vo2x. I can ride at 90% for 90 minutes or more.

If you can't do back to back VO2max sessions on a bike, or maybe do not have an adequate base of training (miles and miles and miles of cycling), you can also consider PM/AM or even AM/PM Brick based VO2max sessions of different disciplines.

As we all know, going out and running a 5k, 10k, half marathon or longer on rested legs is a lot different than trying to do it off a bike. So people incorporate brick workouts where they do a ride/run back to back. Agreed these are great for getting used to how your legs will feel and understanding the importance of your leg turnover when trying to run through your bike legs.

I like doing VO2max work as a brick, with a little bit more rest in between.

Trying doing your run workouts (tempo run, 5k race, track workout, etc) on the morning after a bike VO2max session the night before.

Example:
Monday PM - Bike Workout, 2 hr ride (15 min warmup, 45 min ~65-70% of FTP, main set of intervals w/ 15min or more of total interval time and equivalent rest, 15 min at 70% of FTP, 15 min cooldown).

Tuesday AM - Track Workout (6+ total miles of running - 1.5 to 2.5 mi warmup, 2.5 to 3 mi of intervals 5x800m repeats or similar, 1.5mi cooldown)

I like being able to get out on the track and open it up after already gassing the legs from the bike workout 8 hrs earlier.

Have to find what works best for you....keep a training log of all the data (HR, times, how you felt, etc) so every few months you can look back and adjust your workouts if you aren't getting the results you expected.