I've never used GPU tweak but trixxx seems ok. Its no afterburner but it does the basics well and thats all that matters.
Not sure about about using them both at the same time, you might find as you click apply on one it changes the settings on the other.
If you use trixx though you will be on the un official overclock method. However if its that or no overclocking at all then id just go with it.
yes all 3 effect each other, but Trixx clashes most with AMD Catalyst, you'll notice this if you minimise all 3 together side by side, because if you save one profile in Trixx, you'll see the numbers changing on the software beside it and especially if you fool around with Catalyst.....because as soon as you adjust Catalyst it'll wipe out all your other settings.
thus, i would not have all 3 on your computer at once, only Catalyst; which has to be turned on and right up to max, plus another software, i use Trixx now.
because you cant overclock via Trixx, unless Catalyst is set to max overdrive already, i only noticed this yesterday.... if not, you'll overclock via Trixx, press save and it'll return to default in front of your eyes, leaving you scratching your head like Homer Simpson
Kronkmaster that i was using to benchtest, only stress tests at 1.17 v, i only noticed that yesterday, but it might be ok for you.
so yesterday i said that's enough
and deleted MSI/ KRONK
MSI is ok to use for sure..... but not with Trixx, i prefer Trixx anyway because it's simpler, plus the graphics are far easier to read on a large screen