Hi brandontw, and welcome to The Mattress Underground 
I am working on a new DIY build. I already had a 2" soft 20ILD latex topper on my old bed that i am reusing, then i am getting a 6" HD36-HQ Foam Base (Foam By mail) and a 2" Medium 29 ILD Latex transition layer (Foam By mail)
I’m not sure how it will turn out yet, but i suspect if might not be soft enough, as we are somewhat heavy side sleepers. If i do need more softness, what would be the next step?
I have heard that 2x 2" layers do not act the same a a single 4" layer would, for example…
A DIY project can be the most challenging - but also rewarding! - type of mattress, as it can take a bit of trial and error to fine tune the build; but the ability to so highly customize a mattress to your body and sleep style can result in a mattress more comfortable than one with pre-existing components. And while yes, two 2" layers are not quite the same as one 4" layer, for all practical purposes they feel and act very similar… this should not be a noticeable difference, especially as this allows for greater customization in your case.
The build you describe - a 6" firm foam base, medium firm transition layer, and 2" soft topper
Objectively seems fine; as you don’t provide your Stats but say you are ‘heavier side sleepers’ it’s impossible to tell how this will feel for you…one consideration to keep in mind is that a too-soft comfort layer or an excessively thick one can cause discomfort and issues, OR a too thin layer can cause your hips, for example, to compress the latex foam in that region, sinking down closer to the transition layer.
If i added another 2" of soft latex, possibly talalay on top, would it add the softness we want, or is there another way to do it?
Adding a softer layer on top of a firmer mattress is the simplest way to achieve that. For higher BMI ranges you need to keep in mind that even latex in lover ILD will undergo some mechanical stress and breakdown faster than for a normal BMI. Aside from this there may be a bit of a drawback to DIY – if the comfort support balance is not quite right but as long as you can exchange or switch around multiple layers to get a build that feels ‘right’. You can see a discussion of finishing a DIY top layer in this post and discussion of DIY Layers in this post. There’s really no downside to it but to try this configuration out and see how it feels. Make sure your alignment is straight (your profile should look the same when laying on your side as if you were looking at your back standing up) and try it for a while and see how it feels.
Foam By Mail has I believe a 21 day return/exchange window; as you are using them for the base and support layers hopefully that is long enough to tell if these are comfortable and supportive enough. It is much easier to soften a too firm mattress than firm up a too soft mattress…if this is a not soft enough. Possibly a softer ILD topper will suffice, or a 3" instead of 2". Also, Talalay and Dunlop have very different ‘feels’ - Talalay is ‘bouncier’ and Dunlop more ‘settled’…like the difference between angel food cake and pound cake, so keep that in mind if you need to exchange the topper for additional comfort. Many of the Trusted Members of the site offer latex toppers – for example Arizona Premium Mattress has both Dunlop and Talalay Toppers in multiple comfort levels, and Latex Mattress Factory has Dunlop and Talalay Mattress Toppers as well. We have quite a lot of DIY enthusiasts in our forums, possibly some of them can also chime in with comments regarding your build.
Here’s to many restful nights going forward!
~ Basilio