I've had a 40 litre Engel mounted in my Xtra Cab for a while now and use it as a freezer with an esky for cool stuff but preparing for 3 month extended outback trip last year decided to go for a second Engel.
After many hours of shuffling the Engels around I found that they will fit and are quite useable if both the latches face the middle. There isn't enough room to have both unlatched at the same time, its a tight squeeze to get them both in.
First thing was to strip the rear lining, rear seat belts and all other unnecessary stuff out of the cab and make a 10mm ply wood floor for the Engels to sit on with recesses for the feet to sit in
I decided to use 5mm plastic coated stainless steel wire and turnbuckles to hold the Engels in place -- mainly because one of my previous sponsors had provided it and it was laying around in the shed.
Make side brackets from 50 x 50 x 2.5 duragal angle that bolt through the ply and seat base with large panel washers and nylok nuts underneath and also pick up the seatbelt bolt at the rear.
A bracket to go onto the back of the cab - had to use screws as I cant get washers and nuts behind, not 100% happy with this arrangement but had no troubles so far and done over 30,000km off road since they were fitted.
A bracket to go onto the front of the seat base.
Remove the original Engel handles on the latch side and replace with these
and simply sit the 2 Engels in place and tighten down the straps
They're a tight fit and the outside handlesactually touch the cab lining in the rear corner
Thats how easy it is,
Geoff