You can do that on any multi tuner box using a unicable LNB. Even the original duo could do that, watch one, record one, using unicable. Lots of second hand boxes around with 3 and 4 tuners.
But if second hand is an option, I'd personally try to get a VU+ FBC box instead of an old quad-tuner box.
Can anyone please list VU+ boexes wityh FBC tuners?
Also, what about Dreamboxes? I know they are not supported by OpenPLi but perhaps they could also be considered?