At first glance, it sounds like you are asking for the impossible.
Let me give you an analogy: let's replace "DC offset" ("0 Hz") with red marbles, "20 Hz" with green marbles, and "40 Hz" with blue marbles.
Imagine Bob dumps all of a jar of red and green marbles into the swimming pool.
Then Mallory dumps all of a jar of red and blue marbles in the swimming pool.
Later, we make a measurement at a single point in time and find roughly 22 thousand red marbles, 10 thousand green marbles, and 5 thousand blue marbles in the pool.
So Bob's jar must have had about 10 thousand green marbles in it, and Mallory's jar must have had about 5 thousand blue marbles in it.
How many red marbles did Bob's jar have in it?
It's not possible to calculate it from the given information.
There are several ways to estimate the DC offset of these signals, but all of them require more information.
Off the top of my head, here are a few:
- Somehow measure Bob's red marbles and Mallory's red marbles in isolation, before they are mixed together.
Somehow measure Mallory's red marbles in isolation, and then subtract that number from the total in the pool to estimate Bob's red marbles.
If one of the signals is somehow varying (say, it's connected for a second, then disconnected for a second, then connected again, over and over), then it's "DC" value can be estimated by doing a FFT of the mixture over a very long time, and looking at the the amplitude of mixed signal at the connect/disconnect rate (in this case, the 0.5 Hz bin). The "long time" is necessary to discriminate the 0.5 Hz bin from the ideal 0 Hz bin.
Make several measurements at different proportions. After Bob has dumped in half his jar and Mallory has dumped in a quarter of his jar, stop and count the total red marbles in the pool. Then count the total after both jars are completely emptied into the pool.
Try to measure the proportions of green to red. If you grab a handful of marbles from Bob's jar ahead of time, and you count 5 red marbles and 10 green marbles in your hand before returning them to Bob's jar, you can estimate that there are about half as many red marbles as green marbles in Bob's jar. So since we later find out there were about 10 thousand green marbles in Bob's jar, there must have been about 5 thousand red marbles in Bob's jar.
There's probably many other techniques, all requiring more information.