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People know I really love Kingdom Hearts and its music. I think you did justice to the famous intro theme. Also like how atmospheric this sounds but it doesn't leave the main melody (which is quite simple at its core).

Short concise answers first, and then I'll follow up with technical geeky mumbo jumbo. 1 - Technically, a line output from your audio interface should be able to drive many line inputs because they use Impedance Bridging, which relies on the core assumption that both the line output and line input are done properly. Though to do this does require have cables or a box that allows you to split the signal. However, if you have multiple outputs then it affords you more flexibility in how you can set things up. 2 - I don't see any reason why not. Just well made cables I suppose. You know for cable longevity and reliability. All right time for the technical geeky mumbo jumbo if you want some further explanation on things. Lets talk about that headphone output on the Roland Rubix 24, which Roland actually gives a maximum power specification (well at least sorta kinda, but better than nothing). Anyway, so a headphone output is really just a mini power amplifier because at the end of the day all a pair of dynamic headphones (like the two you listed) are just tiny speakers. So, Roland specifies the output power as 20mW into 47 ohms. You can estimate the amount of SPL your headphones will have based on this. It isn't perfect and you could sit down and try and do fancy mathematics to get at a more concise answer, but this is just quick and dirty. However, simple cross multiplication should "ballpark it". So, for the Sony MDR-7506 headphones you can expect that about 27mW maximum will be on tap. This is calculated by the classic (20mW/47 ohms) = (X/63 ohms). From here you get (63 ohms * 20mW) / 47 ohms. The ohms cancel out and you're left with 26.8mW. At any rate the MDR-7506's have a specified sensitivity of 106dB/W/m. This would give a peak SPL in excess of 120dB SPL. So, I'd say there is probably plenty of room there. Next up are those Beyerdynamic DT 880 250 ohm headphones. So, these are rated at 96dB/W/m (not stated like that, but at any rate). So, doing the same trick as before the Roland unit should produce around 106mW into the DT 880s. This gives a peak SPL around 116-117dB. Now, these numbers are estimations on the maximum SPL that you can expect from the Roland and can be eaten up quite quickly by very big and powerful transients. So, unless you want more volume I've never really understood the point of headphone amps 99% of the time personally, but I digress. Most of the time headphone outputs simply use an inadequate device for the output stage without enough bulk energy storage (big capacitors on the power rails). My best guess is that the Roland is probably using an "okay" opamp as the output stage. I'd be willing to bet that the headphone output on the Roland is either a single JRC4558 or two JRC4558s that are paralleled up. I doubt that they're using anything better, and for a whole mess of reasons this part is basically barely & adequately passable part for the job, but again I digress. This then I guess begs the question why would I say much about that? Well see the output is probably just an opamp anyway. This means that it'd have no problems driving any line input. It won't be balanced and it will probably be noisier than a dedicated line output given that it is being used as a power amp, BUT there is nothing "technically" wrong with using it that way. Not best practice of course. I also feel it necessary to say something about the impedance balanced output. Really the trick here is to understand what "impedance balanced" actually means. In a nuthsell this means that the cold lead or "- lead" is not actually driven by anything, but instead presents the same "impedance" to ground that the hot lead or "+ lead" presents. This will eliminate most things like ground loops and hum, but not much else, which for most things is an acceptable compromise in cost. The other common option is to drive both the hot and cold leads and this provides much better rejection of all sorts of things, but it is more costly. Why use two opamps, four bypass capacitors, and five resistors when you can use one opamp, two bypass capacitors, and six resistors. Takes less board space and costs less overall to do the single opamp. Hopefully, you found something useful in there.