Doh - excuse my rambling as I thought you wanted to move him to another room entirely. Well, most of it still applies. I'll just add:
The research doesn't specifically say that the risk of separate rooms applies to all sleeps, and there is no official body that I know of anywhere in the world (including the NHS, BTW) aside from The Lullaby Trust which recommends this. And in fact the Lullaby Trust have only begin to recommend this in the last few years. It's a fairly recent addition by them, and I'm not aware of any new research which has prompted its addition.
Now, you could take this one of two ways. We don't have research which specifically contrasts shorter sleep periods (naps, early evening sleep) with the longest stretch of sleep that tends to overlap with adult sleep, which is what most advice centres on. Common sense would say: OK, SIDS can occur during any sleep, so it makes sense to follow guidelines during every sleep. Fair. However, as you're finding, in reality, not so practical. And in real life where people aren't perfect you have to weigh up the cost and the benefit. So if you have a full term, healthy, vaccinated, not underweight baby and neither parent smokes or is a drug/alcohol addict, you're already pretty much as low as you can go in the SIDS risk stakes. If you breastfeed, even partially, even lower. So if you have a baby who is already very low risk for SIDS, do you really need to be worrying about following every guideline to impractical levels during every sleep? Realistically - probably not. OK, don't take that as a green light to go and decorate the baby's cot with pillows, whack the heating up to full and put them to sleep on their front, next door. That would be silly. But putting them to bed before you in the evening so that you can actually have an evening before you go and spend that majority portion of the night where your sleep times (mostly) overlap following the guidelines more completely? This is not high risk for these babies. And in reality it's probably not a huge deal for any baby, unless they are being put to bed in an unsafe environment. We are talking 2-4 hours out of 10-12 - it's not a massive proportion. Babies of an age to be high risk for SIDS are typically waking very often to be fed or soothed back to sleep, anyway. Your mental health and time together as new parents is important. Establishing sleep habits that don't massively disturb the baby is helpful. I don't think the Lullaby Trust's advice is common sense. I think it's confusing, albeit technically correct. And I also think the type of parents worrying about breaking it are not the ones whose babies are most at risk.
It's unsupervised sleep, so all of the suffocation and entanglement type risks still apply, but with a clear cot (no sleepyhead etc), baby on their back (unless they can roll to their front by themselves), not swaddled and especially over 4 months old, I would not hesitate to do this. We did it at 3 months and I didn't worry about it.