Simple answer: extra investiture for compounding comes from Preservation.
Long answer: The mechanics of that are a little more complicated; but it's not too hard to understand once you understand the differences in the systems. With the metallic arts, you can classify the three systems as end-positive, end-neutral, or end-negative (source).
Allomancy is end-positive - the power for allomancy doesn't come from the person/metal. The metals just act as a key to Preservation's power. End-positive references that the use of allomancy basically adds Preservation's investiture into the physical realm from the spiritual.
Feruochemy is end-neutral - the power for feurochemy comes from the feruchemist themselves. No additional investiture is added to the physical realm, it is just stored in the metal without gaining or losing charge.
Hemalurgy is end-negative - when making a hemalurgic spike, the power that you steal is diminished compared to the person it is stolen from. Some of the investiture is lost from the physical realm. This fits with Ruin's want for things to decay.
So understanding all this, we can fairly easily explain how compounding works. With feruchemy, you are only able to store the investiture you have access to; it doesn't provide any key to a shard's investiture. Allomancy does provide a key to Preservation's investiture though. When a twinborn burns a metalmind with a feruchemical charge; they are able to key Preservation's power to that charge, and use their feruchemical abilities to store that additional power in a metalmind. They are able to convert the end-positive energy from allomancy with the end-neutral nature of feruchemy. This allows them to amplify feruchemical abilities, AKA Compounding.
See also this explaination from Brandon Sanderson (emphasis mine):
Ok, last question. It was really difficult coming up with three questions that haven’t been asked already...
OK... you’re not going to ask me the “what would you ask me” question?
OK good, because I hate that one! (laughs)
My question is if there’s anything that you’ve never been asked that you would like to talk about?
Oooooh, ok. Hm. That one is so hard! Every time people ask me something like this... What have I never been asked that people should be asking, is basically what the question is? Something that the fans have just missed... They pick up on so much, that it’s hard... I do wonder if, you know… all the magic systems [in my books] are connected and work on some basic fundamental principles, and a lot of people haven’t been asking questions about this. One thing I did get a question on today, and I’ll just talk about this one... they didn’t ask the right question, but I nudged them the right way, is understanding that tie between Aondor [the magic system from Elantris] and allomancy [Mistborn’s magic system].
People ask about getting the power from metals and things, but that’s not actually how it works. The power’s not coming from metal. I talked a little about this before, but you are drawing power from some source, and the metal is actually just a gateway. It’s actually the molecular structure of the metal… what’s going on there, the pattern, the resonance of that metal works in the same way as an Aon does in Elantris. It filters the power. So it is just a sign of “this is what power this energy is going to be shaped into and give you.” When you understand that, compounding [in Alloy of Law] makes much more sense.
Compounding is where you are able to kind of draw in more power than you should with feruchemy. What’s going on there is you’re actually charging a piece of metal, and then you are burning that metal as a feruchemical charge. What is happening is that the feruchemical charge overwrites the allomantic charge, and so you actually fuel feruchemy with allomancy, is what you are doing. Then if you just get out another piece of metal and store it in, since you’re not drawing the power from yourself, you’re cheating the system, you’re short-circuiting the system a little bit. So you can actually use the power that usually fuels allomancy, to fuel feruchemy, which you can then store in a metalmind, and basically build up these huge reservoirs of it. So what’s going on there is… imagine there’s like, an imprint, a wavelength, so to speak. A beat for an allomantic thing, that when you burn a metal, it says “ok, this is what power we give.” When it’s got that charge, it changes that beat and says, “now we get this power.” And you access a set of feruchemical power. That’s why compounding is so powerful.