"...Would you use tallow and lard together in a recipe or would it be better to use one or the other? For example I have a recipe that is 30% Lard. Could I split it 15% lard and 15% tallow? Would I see any benefits?..."
I'm a newbie to tallow too -- I've used it in just two batches so far. What I've seen so far is tallow does bring something to the party that lard does not. I don't know that I would completely replace the lard with tallow, but I can see the benefit of using a moderate % of tallow in a recipe.
A high lard recipe (80% lard, 15% CO, 5% castor) that goes through gel during saponification is firm and ready to cut in 12-18 hours. The feel is waxy and very slightly pliable like a refrigerated cheddar cheese. The soap just barely yields to hard finger pressure. It is easy to cut.
I replaced 15% to 20% of the lard in this recipe with home-rendered tallow. The soap is firmer yet, even tending to be brittle, with a dry texture more like ice-cold butter. It does not yield to hard finger pressure. It may have to be cut much sooner, so it doesn't shatter during the cut -- or it may need to be warmed in the oven to soften it enough for cutting. The lather is much nicer than I thought it might be with that much added tallow -- that was a concern I had about this recipe before I tried it.
I think if you add tallow to a recipe that has a lower % of solid fats than the recipes I'm using, the tallow may add hardness to the soap, but the soap may not get as brittle and may be easier to cut. That's just an opinion -- I haven't tried this to know for sure. But if I were you, I'd sure try making one batch with 15% each lard and tallow and see how it turned out.