I tried yesterday to set up multilink over 2 X.21 links with the same bandwidth.
You just have to get rid of IPs, ACLs etc in the Serials and create a Multilink1 interface. You can config this interface with IPs and ACLs. The trick is that you have to conf the serial and the multilink, giving the commands:
multilink-group 1
ppp multilink
Also you have to check your routing (especially statics routes)
To be honest it did not work for me. I had packet loss for packets over 200 Bytes that i could not find out where the problem was. Someonte told me about ppp fragmentation...