Workout length. Sessions on the erg longer than 30min have been linked to lower back issues. Some college programs limit their rowers to 30min sessions. Some limit to 30min pieces with stretch/rest breaks. Some do 60-90 min erg sessions. Use your own best judgment, but bear in mind that if your technique gets spotty while you’re fatigued, you could be setting yourself up for injury.
Gross mechanical efficiency. Some athletes who have a lot of experience in both sports can match 1-20min power on the bike and the erg. Most of us cannot. Not even close. Don’t try to match bike power on th erg. Better to go by HR.
You can put rowing on top of cycling, but go easily. If your legs are fatigued from cycling, trying to do much more than steady, nose-breathing endurance work will probably lead to cheating with your back to compensate for leg fatigue, or you’ll start pulling with your arms too early, or other technical errors that will send load to your back muscles and spine.
Given the technical demands of getting the erg stroke down (which is not nearly as real rowing!) and the possiblity for injury, I’d say just use some rowing sessions as low intensity calorie-burning filler. After a few months getting the technique down and gaining some base fitness on the erg, maybe substitute a tempo session of the bike with some upper HR zone 2 work (what rowers call UT1), but I don’t think it would be a good idea to try threshold or VO2 (what rowers would call Transport) work. Too much chance for tweaking your back for too little gain, I’d say.