Parts of the Philippines are wired like the USA. But they are trying to harmonize their power grid to the EU style, so people in those zones must wire like North American 240V-only - with 120V and neutral use forbidden. At some point they will move a jumper on the supply transformer and be full EU. If you are in the Philippines, your plan is fine. Stop reading: this answer is over for you. For everyone else in El NEC territory:
Neutral vs ground matters
Neutral is not ground. It's never ground. It's not even a little ground. Criss-crossing them or using one for the other seems harmless, but it actually creates a time bomb that means a simple loose wire somewhere could kill you. They must be absolutely 100% separate at all times, anywhere but the main panel.
Here is the root problem with 240V-only panels. Sooner or later, somebody will want to stick a 120V circuit in there. Or a 120V/240V circuit (primarily 240V but uses 120V for controls, e.g. a dryer or range). They won't even think about what they're doing; this will seem very natural since you do exactly that in a main panel. At this point, a simple wire problem will create a lethal situation, as above.
Because of this, inspectors generally want you to wire the neutral even if you're not using it, to protect you from that future mistake.
You might as well. You're wasting a wire if you don't.
You already have enough wires to deliver 2 hots + neutral + ground. The trick is, EMT conduit is an acceptable ground path. So the conduit itself delivers safety ground; your smaller wire delivers neutral; and the 2 larger wires deliver hots. Everybody's happy.
You will need to spend $7 on an accessory ground bar for ea--- wait a minute. Didn't you say the entire building is wired in EMT conduit? You're already finished with grounds. EMT is your ground.
Seriously. This is how commercial wiring is done. I have four buildings with 7 panels with 6 services, all done in EMT or busway. None of the panels even have a ground bar. They've been touched by a dozen electricians and many permit/inspection cycles, and nobody's batted an eye or said a word about it. About half of the work is old enough to predate mandatory grounding, and half is post-1980; there's no difference in wiring method. It's all Code legal today except for post-2011 AFCI/GFCI requirements and I refuse to put handle ties on certain breaker-switched, lighting-only MWBCs.
The only time you would ever put a ground wire in an EMT conduit is if you had an expectation that the conduit might take physical damage hard enough to part a coupler. But at that point, you really should be not using EMT there, either better protect the EMT or run Rigid.
Cable in conduit
Is legal, but you'll need a good supply of swear words to pull it. It will be difficult to pull, and we like to warn DIYers both a) about the difficulty of pulling cable, and also the difficulty of any pull that is near the capacity of the conduit. Generally, cable inside conduit should always be avoided; run THWN or XHHW single conductors to suit. But since you already have this....
... Well, hold on. Does your USE cable have markings on each individual wire? Does it easily separate? If so, you can easily separate it, and get to play by the individual-wire rules. Otherwise you're stuck with what follows.
When you have a lumpy or non-round cable, you must treat that (for conduit fill calculations) as a round wire of the large dimension; i.e. what circle will this cable fit inside? CME Wire says 1/0-1/0-2 URD is 1.14" in diameter. Okay. With one wire, the conduit's inside diameter must be 138% of the cable diameter. (technically, that's 53% conduit fill, but what I just said is easier to compute). So for you, 1.57", but you just make it with 1.5" trade size conduit.
- At 2 cables, you're only allowed 30% fill. So now the conduit must be 259% of the wide dimension to support 2 identical cables. (e.g. 3" conduit for dual 1/0-1/0-2 URD. Ouch).
- At 3 cables, you're allowed 40% fill, so for three identical cables, 274% of the width of one cable. (still 3" EMT).
- At 4 cables, conduit derate becomes a concern, but not for your oversize wire. Conduit ID must be 317% of cable width. For you, 3.5" trade size EMT.
- At 5 cables, conduit derate becomes so severe that it's not worth doing.
Adding an extra ground wire inside the conduit means that would be two "wires", forcing you to 2.5" conduit. Ridiculous. So don't do that; secure the EMT firmly or run a ground wire outside it.