But still naps downstairs, what do they sleep in? I have a travelcot but it's huge! DS1 slept in one of these but now I'm afraid DS1 jumping around while DS2 tries to sleep might not work. He's 10 weeks old now but pretty huge.
I put DS in his crib for all sleeps when he was too big for the moses basket. I kept the door open and monitor on. Saying that, at 10 weeks he didn't like the moses basket much preferably surgically attached
When convenient, I put DS down on a woven blanket on the ground in a dark and quiet room e.g. in the living room with the curtains drawn if I'm working in the kitchen during nap time. At that age, he was too small to roll over. I probably stopped doing it around 4 mo.
If DS1 needs to jump around and play without constant reminding of keeping it quiet because baby is sleeping, you might be just as well off putting DS2 in his cot upstairs with the monitor on. I started doing that probably around the 10 week mark as well and developed a little nap time routine which we still use now; it helped with the eventual transition to overnight sleep in his own cot. So sometimes he'd sleep in the cot and sometimes downstairs on the blanket - but either way, somewhere dark and quiet where he wouldn't be disturbed by me singing along to the radio while cooking