LAG port(s) connection between 2748 and 2848 not working

i was running two 2748s, in managed mode.  one is in the rack connected to the servers, wifi WAPs and internet infrastructure.  one is connected to the wall ports in the building.  the one in the rack recently broke down.  it looks like 2 or 3 of the larger capacitors (2200uF/16v) in the power supply have gone bad (the "X" on the cap's top have split).  while i'm debating the feasibility of just trying to replace those caps, i've managed to procure a surplus 2848 from our "room where old equipment goes to die".  configuring the 2848 was easy enough to replicate the 2748's IP, VLANs, LAGs, etc. 
in the previous configuration, i had ports 45-48 configured as LAG-1 and used it as a 4gb/s trunk from switch to switch. 

i did the same on the 2848, but the problem is there's no communication between the 2748 and 2848.  traffic flows (ping tests) between workstations on the 2748.  traffic flows between servers on the 2848.  nothing on the 2748 can see anything on the 2848 and vice-versa. 

am i missing something in the LAG group configuration on the 2748, 2848 or both in order to get them to talk to one another?

i really don't think the answer is "you have to use two 2748's or two 2848's".

i unplugged one end of the LAG connection and just connected the two switches over a pair of VLAN-1 ports and everything is fine, except flowing at only 1gb/s, instead of 4gb/s, going switch-to-switch.  i want that higher throughput (as well as the redundancy from using multiple ports). 

what am i missing?
Thanks