Is this a Monash question or a uni question in general? If so, I will move it to General Uni Discussion.
Overall - a double degree does take up more time and energy than a single degree. But of course, there are benefits to putting up with it (two separate degrees at the end for 1-2 years more). However in saying that, usually with a double degree you do somewhere around 5 units a semester. While this doesn't sound like much to you at the moment, it can take its toll. It's quite difficult to maintain 4 units, let alone 5. I would
personally only be opting to do a double degree if both degrees have meaning to you and will be useful in your later life. One example that I can think of that is a good combo is the Bachelor of Arts and Bachelor of Education (they mould well together for obvious reasons) - certainly not limited to this though.
In terms of employability - I really can't comment on that because I'm not in this area, but i'm sure someone else will elaborate soon