Wearing two wetsuits to keep warm… well you can do it but it depends on what are you doing. If you want to use two wetsuits, one over the other for a very active sport like surfing then two wetsuits will be stiffer than one wetsuit of the same thickness. If you will be using them for some more static activity where flexibility is not that crucial then go for it, no problem. Also one thicker wetsuit would be warmer than two thinner combined. This is especially true if the one that you wear on the outside is not tight enough and water gets in between both suits.
It also depends on what wetsuits are you combining. You can easily put a wetsuit vest under your wetsuit. Wetsuit vest has no arms, it will only thicken the neoprene on your body where flexibility is not a problem. So this is a great idea. Lots of people also weat a wetsuit top under their wetsuit to add an extra layer of neoprene to be warmer. Since wetsuit tops are usually very thin… like from 0.5 to 1.5mm this also doesn’t make much difference in flexibility.
Anyway – you are probably asking this because you don’t have a thicker wetsuit and you want to go into cold water. There is nothing wrong with wearing two wetsuits, you will see for yourself if this feels too uncomfortable and if it limits you so much that you cant enjoy your water sports. People do it all the time. It still beats not going into the water at all, right:)?!