As already noted by ShadowKras and Kommissar, you can only use the 'Focus Power' Action once per regular Round. This means you can only do a normal spell-cast once per Round.

One thing to remember, however, is that while you may only be able to cast one Psychic Power per Round as per the rule stating you cannot use the same Half Action more than once per Round (in this case the 'Focus Power' Half Action), you can sustain some powers over more than one turn. A Power that can be sustained lasts for 10 Rounds before needing to *be* sustained, which is affected by simply passing a Free Action 'Focus Power' for that Power. Furthermore, the only penalty for trying to cast and sustain multiple powers in 1e is that you get -4 to Power Rolls to cast while sustaining 1 spell, -8 while sustaining 2 spells, and -16 to cast while maintaining 3 spells. You can't sustain more than 4 powers at once.

Here's the entry regarding sustained Powers from page 164 of the Dark Heresy 1e Core Rulebook:

>**Sustaining Psychic Powers**

>There are a number of Psychic Powers that are worth “keeping on” for an extended length of time. Powers that can be used in such manner are referred to as being “sustainable”, which is noted in their entries. Sustaining a Psychic Power for an extended period of time is very taxing and only the most proficient Psykers can accomplish it. Every 10 Rounds a power is active, the Psyker must make a Power Roll to maintain it—just as if he were rolling to manifest it. Unlike a normal Power Roll, this is performed as a Free Action at the start of the Psyker’s Turn and cannot invoke either Psychic Phenomena or the Perils of the Warp.

>**Sustaining Multiple Powers**

>A Psyker can sustain a single Psychic Power without much effort once properly activated. However, all Power Rolls the Psyker makes while sustaining a single Psychic Power are reduced by 4. A Psyker can sustain multiple powers, but all Power Rolls get progressively more difficult as he must expend more and more of his power on maintaining the powers already in effect. A Psyker sustaining two powers reduces all Power Rolls by 8 while a Psyker sustaining three powers reduces all Power Rolls by 16. It is not possible to sustain more than four powers at once. These penalties to the Power Roll also apply to sustaining powers already in effect.

This is why powerful psykers are scary; they can buff themselves and/or others, debuff one or more enemies, and/or throw out insane amounts of armor-bypassing damage per Round.