As I see it, there are a couple different things going on in this question, which I'll deal with in turn. The first two are unrelated to seventh chords.
- There's a confusion about the meaning of tonic.
- There's a question about the similarities of relative major and minor scales.
- There's a question about seventh chords being viewed as two chords combined.
First and foremost, you seem to be mixing the concepts of chord, and key. Specifically, the word "tonic" always relates to the key you are in, regardless of what chord you are playing. It is the tonic that describes where the melody wants to rest, or resolve to. On the other hand, terms like "root" and "third" could be used to relate to either scales, or keys, and the context should make it clear which is which. In other words, there are two possible reference points for counting intervals, and you seem to be confusing them.
This actually has nothing to do with seventh chords. For example, your Cm chord could occur in lots of different keys. Here's a few:
In the key of C minor:
- The tonic is C.
- The root of the Cm chord (C) is also the same as the root of the key (tonic).
- The third of the Cm chord (E♭) is also the the third of the key.
In the key of E♭ major:
- The tonic is E♭.
- The root of the Cm chord (C) is the sixth of the key.
- The third of the Cm chord (E♭) is the root of the key (tonic).
- Note that this partly explains why E♭ can sound like a good tonic
In the key of A♭ major:
- The tonic is A♭.
- The root of the Cm chord (C) is the third of the key.
- The third of the Cm chord (E♭) is the fifth of the key.
In the key of B♭ major:
- The tonic is B♭.
- The root of the Cm chord (C) is the second of the key.
- The third of the Cm chord (E♭) is the fourth of the key.
So as for which note is the "tonic" (i.e. the note that melodies should resolve to)? It really depends on what key you're playing in.
Why do C and E♭ both sound convincing as tonics when playing this scale? You'll notice that the E♭ major and C minor (natural minor) scales share the same key signature of 3 flats. This also means they share all the same notes of the scale, but with different scale degrees (for example, the notes F-G-A♭ could be viewed as scale degrees 2, 3, 4 in E♭ major, or as scale degrees 4, 5, 6 in C minor). These keys are thus considered related to each other. You would say that C minor is the relative minor of E♭ major, or swap it around and say that E♭ major is the relative major of C minor. These same terms can be applied to chords in addition to keys. Every major scale or chord has a relative minor scale or chord that starts a minor third below it (and vice versa). Once again, this is unrelated to seventh chords.
See: Are the formulae for Minor scale and Major scales related?
Finally, you have a very interesting question that does concern seventh chords -- can seventh chords be viewed as two superimposed chords?. For example, can a chord comprised of 1-3-5-7 be viewed as a combination of the chords 1-3-5 and 3-5-7?
I would argue that the answer is yes, to a limited degree. It doesn't affect what the tonic is, since that's determined by your key. Nor does it change what the root of your chord is. But the chord's "quality" is definitely affected by the quality of the upper chord. Let's look at some examples. For each seventh chord below, try playing the two triads separately a couple times, listening to their differences, then play them together as a seventh chord, and here how they both contribute to the resulting sound.
Dominant Seventh = Major + Diminished
- e.g. (G, B, D) + (B, D, F)
- This chord combines the boldness of a major chord, with the instability inherent in the diminished chord, resulting a chord that demands resolution -- so much so that it practically defines the whole concept of western tonal music.
Major Seventh = Major + Minor
- e.g. (C, E, G) + (E, G, B)
- The outgoing happiness of the major chord is somewhat tempered by the sorrow in the minor chord, leading to a sound that could be described as bittersweet, melancholy, or even a nostalgic.
Minor Seventh = Minor + Major
- e.g. (C, E♭, G) + (E♭, G, B♭)
- This intense sorrow possible in a minor chord is significantly reduced and stabilized by the major chord leading to a rather laid-back, mellow or chilled-out sound. The theme from Beethoven's 5th symphony sounds dramatic over a minor chord; over a minor seventh... meh, not so much.
In each of these cases, you can think of the upper "chord" as somehow flavoring the perception of the lower chord.
Update: As @Ceart mentions, the more general term for the superposition of two different chords is a polychord. Because seventh chords are so ubiquitous, I don't think they are usually thought of as polychords, although technically they could be. When seen from that standpoint, here is a related question: Difference between Polychords and Extended chords