Book contents: mix and match?

Traditionally, the DM treats books as sources of ideas; maybe following closely with a campaign, maybe not.  I'd like to be able to combine one book with another.  Is there a recommended way for approaching that idea? For instance, suppose I purchase both "Out of the Abyss" and "Storm King's Thunder".  (I haven't yet.)  The second book is geared toward being a campaign, or providing pieces to mix and match, and I'd like to use it that way. I'm well versed in toting characters around, and my PC's can refer to source books I've purchased, but not so much on the campaign side.  Is there something I'm missing?

Keeping to Roll20 advice, combining two purchased modules from Roll20 would mean you'd need to create a game with each, and them use the transmogifier to move content you want to your combo-game. It would likely be a bit of work, so you'd likely want to plan beforehand how you do this before you start moving things around on Roll20. If you own own one or neither module on Roll20, there is no problem as you'd just create the game with the module you have or just a clean game, and them manually convert your content to the Roll20 game when you need it, not even requiring access to the transmogifier. If you're looking for general gamemaster advice on how to combine two campaigns that aren't directly related to Roll20, I advice you to check communities dedicated to 5E.

To add on to what Andreas said...If both modules are Add On style modules (for example, Sunless Citadel) then you can just add them into an existing campaign and delete the content you don't need from both modules. I don't know anything about those two modules, so can't tell you what type of module they are but between what Andreas said and Add Ons, you should be able to do what you want.

Finderski said:  I don't know anything about those two modules The two mentioned by OP are specifically full Roll20 Modules and not Add Ons , so I didn't feel I needed to explain the alternative. But this reminds me that Out of the Abyss have a Compendium section & an art pack, which makes it much easier to quickly add content from to other games. And I just realized Michael J. is only a Plus user, and doesn't have access to the Transmogrifier , so my suggestion won't entirely work. However, if you gone the mile to buy both games and are dedicated to combine them for a very long campaign, I'm pretty sure trying out at least for a while to get Pro for the transmogrifier would be worth it. So if OP have bought both Storm King's Thunder and Out of the Abyss on Roll20, and you aim to combine them more or less evenly, my revised suggestion is to create one game each for SKT and OotA, and then make a duplicate of of SKT to use as a base for your game. Then import things when needed from OotA to the main game, and remember that you can get it's monsters from the Compendium which cuts down amount of things you need to transmogrify(or manually recreate/convert if you don't go Pro). As your game progresses and you start to add more and more own or OotA content to the game, it's likely a good idea to eventually start removing or transmogrify content your group won't revisit or need, as campaign with a lot of content will eventually start getting performance issues, and having the content of two large 5E modules in a single campaign surely will be noticeable. The best would be to have a dedicated "storage" campaign where you offload stuff you're sure you won't need in a good while, but want to save anyway.

Thanks, both. True, I haven't bumped up to Pro yet, and that's not likely to happen in the near term, so I had forgotten that the Transmogrifier option existed. Currently my first party is midway through the Phandolin adventure, and it doesn't have the mix-and-match modules that other packages do.  (Right?  I'm not overlooking something?)  Only since I posted my question did I notice that some other packages offer that function. My question is answered, but in case my perspective is worth noting: Yes, I'm coming at this from an "in Roll20" perspective, that's why I asked here. Now that I have my feet wet, my hope is to draw a world, with plot taken from random sources, but the work involved online is, in a way, doubled from what it is at a table.  In person I could skim (rob) from the Dungeons & Dragons books or anywhere else, without being obliged to follow the plot line.  "Storm King's Thunder" was written with that in mind.  But, the online system seems to assume that the DM draws the plot from there as well.  I understand why the developers here would be wary of making pieces easy to haul around (see above, "rob") but I'm not really interested in giving my players what amounts to a slower version of an online RPG.