Cheers in advance for any help guys...
Yes, you're confusing a couple of issues here, but let's clear them up.
Firstly, the twin sockets on the cabs (and, indeed, on almost all amps...) are wired in parallel. One can use either; it makes no difference. If an amp has two outputs, one would usually plug a cab into each, and the amp would 'see' the total impedance in parallel, just as if there was only one cab. For the two 8 ohm cabs mentioned, the amp would see 4 ohms.
Your amp has only one output, so you would plug instead one cab into that, and the other into the second output on the first cab. The result, electrically, for the amp is identical; it will see 4 ohms just the same.
The power handling question is also slightly askew. Each cab has a rating of 400w. Its impedance is of no importance for that; it just 'is'. The amp has a power output dependant to some extent on the impedance presented. It will give a certain power at 8 ohms, and a bit more into 4 ohms. The power rating of the two cabs combined is 800w. The amp will be able to supply 500w into that load (4 ohms...). The cabs are therefore 'safe'. One single cab can handle 400w, but the amp, into 8 ohms, will be able to supply only around 300 or so, so, again, the cab is 'safe'.
A word of warning, just the same. The manufacturer's rating of 400w doesn't mean that much in real terms; it basically means that the speaker (probably...) won't burst into flames before that wattage. In reality, it may very well sound bloomin' awful well before then. Use your ears to judge if the cabs are being over-driven (and wear ear protection, if you value your hearing...). Don't wait for the magic smoke to appear.
There; does that help at all..?
Subject to completion, correction and/or contradiction from others.
Edited by Dad3353, 10 June 2016 - 07:21 PM.