Its possible only rear facing. I only load from the side door for my 4 year old Son. Actually, its even easier than that. He gets himself buckled, I just tighten the straps. However, I have not had two full size rear facing car seats in use at the same time, my kids are about 3 years apart, so you may have had a different experience than me.
One of the reasons I made my cargo box in two parts was so that I could easily remove half to load my Son into his rear facing car seat.
From what I can deduce, mindaugas is about to purchase a vehicross, but he has a little voice telling him its a bad idea because they either have a small child, or are about to have a small child. My wife was pregnant with our first child when I sold a 4 door vehicle and purchased a 2-door vehicle (vx) so I have been in those shoes before. I was greatly surprised by how well I could make it work for my needs.