It’s usually fine to just use adaptor splitters or Y splitter cables, with no degradation of note. Electrically that’s the same as preamps with dual outputs - it’s just a convenience to have the extra outlets right there.
If you find a situation where it causes problems, then that preamp was probably already marginal into a single amp. What you want to avoid is a pre with high output impedance (> 1K ohms) into amps with low input impedance (< 10K ohms, or < 20K ohms for dual amps). That’s more likely to occur when mixing tube pre with solid state amps, but in most cases even that is fine. The last few tube preamps I’ve owned have had output impedances <= 300 ohms, and will drive pretty much anything (even twice) with no problem.
Just be aware some amps spec a much lower input impedance on their XLR inputs versus RCA, in case that becomes a factor. Though rare, you could find use cases where RCA is the better choice, based on that alone.