Hi,
yes this is possible. F.e. CE1(3620) - PE1(2500MPLS) - PE2(2500MPLS) - CE2
In case you have Serial interfaces use Frame Relay with different, separate PVCs and you can also setup "redundancy" and the like.
If your 3620 and 2621 IOS supports tag-switching you could use them as PE routers. There is no need for a "P" router to test MPLS VPN.
Hope this helps! Please rate all posts.
Regards, Martin