Different start times?

I guess it depends on if they are autos, or photos. If autos no really issues besides maybe space. If it's photos, that entirely depends on if the younger plant is ready to flower by your standards

If their photos and one needs or is able to flower and the younger one is not, you'll need another room for the extra 6 hours the one still vegging will need until its mature enough to flower. Cheers

it is just a different harvest times. if you dont have for a plant to hang. i prefer to different start times then i can have different harvest times. maybe not disadvantages just it is more work if you have one in flower and one still in veg. but if you want more and a constant harvest then different start times

The same genetics cause one plant become bigger and higher, u can use lst to bend the canopy at the same height , one plant will take more space, u have to chose the right time to flip them in flowering to give them all the space they need. With different genetics u can’t know for sure, the second plant can be bigger than the first or very tiny.