Can you use Hammer blow twice if you do two all out attacks granted by the devastating assault talent?

Yes. In fact the way the rules it implies you must do it, as devastating assault says you must make the exact same attack twice. Or paraphrased 'with all the same penalties and bonuses as the first".

At home with my book open so I can give you a more proper readout.

Devastating Assault pg.125

An all-out attack that is successful "the character may make a second All out attack action against the same target as a Freee Action, with the same bonuses and penalties as the first"

Hammer blow pg.128

"When he uses an All Out Attack action to make a single melee attack, he can add half his strength bonus (round up) to the weapons penetration. The attack also counts as having the Concussive (2) weapon quality, to represent the shocking force of the blows impact"

To me, it seems like a simple yes. Both hits will be Hammer time. Though nit picking with the wording a GM could say that the talent implies that only the base form of all out attack can be used with hammerblow, but I have always assumed and the book implies it as well, that talents always override the basic actions.

Well, I found where I am building my PC to next. Time to crush the hell out of everything with an eviscerator.

Warhammers are pretty great too. Get a best quality Warhammer, (talk to your GM or have your local techpreist work on it), upgrade it with reinforced, mono, modified stock, and a custom grip, for some sweet combat effectiveness. It is best talk to your GM to get some extra bonuses and perks. A good idea is have a backstory for this weapon. As a GM I'd give a wary stare if a player just come up to me and said "I want this, this and this. And I roll my requisition test." However, if they came up to me with a kick ass drawing (or a feeble attempt for those who don't have art skills) and a backstory/history and a fleshed out concept for a weapon, I might even give bonuses to their requisition test. Also a warhammer, you can stack its concussion with your talent. So a Best Warhammer (stock) add some talents... 1d10+3+SB+WSB/2+PB* concussive (1) And you'll hitting hard with concussion (3) A freakin' -30 toughness test!!

With all the upgrades I mentioned prior you'll have a weapon profile akin to this 1d10+5, Pen 4, Balanced, Proven (3), Concussive (2), Mono, +15 WS.

Stack that with the talents and your bonuses it is nothing to sneeze at. Heck, if you really wanna make it a jerk of a weapon get a Tox dispenser** just to give it that extra "screw you" to any baddy.

*Crushing blow, Deathdealer, Hammerblow, Devastating Assault

** Gives a -20 toughness test or take an extra 1d10 damage; ignore armor.

Many people like to grip and complaint that the upper teir weapons completely outclass. I disagree, with some imagination, some decent character build and a few upgrades to a low tier item, you can have something which will perform nearly as well as a powersword. (unless it parries you of course...)

... All being relatively cheap XP-wise with decent aptitude combination, and pretty easy to requisition given the availability of those items. Yay for stun locking!

Don't forget to be a feral worlder for added smashin'