Should have realised that I could just search in a relations file to find an answer to my question

Here's one from a job with lim1=175M, lpb1=32, mfb1=90:

178218979181,5545:2,b,fb,445,3b21,3c47b,166eeb,b38e5f,c2091937:3,3,3,5,71,89,265,9e3f5,c5a6b,451d0b,1c9c589,45458a5,f085ca3b,fb78b42f
That's two large 32-bit prime factors on the algebraic side, so the cofactor wasn't far off 2^64. It sure looks like CADO is just using a bound of 2^(2*lpb).

It feels like there could be a decent speedup from setting the 2LP bound separately in this situation, and I'd be surprised if no-one had thought of this before, so is there some reason why this wouldn't actually work in practice?