Knowing you can combine 2 signatures, Is it possible to remove part of the signature? Since my last post I've been more interested into RSA cryptography, but I'm questioning one thing.

Lets say we are using standard textbook RSA with no padding and with the standard signature generation in lets say the Microsoft crypto API and you got the part you want to remove by itself is it possible to remove that from another signature with the same thing on the beginning or end?
As soon as you are working using standard textbook RSA without padding (or hashing), you can fully exploit the malleability of the RSA cipher.

So, let's say you have a message $m$ which is equal to $a\cdot b$, for $a,b$ two values you know. If you can get the signer to sign both $a$ and $b$, you can compute the value of the signature of $m$, thanks to that malleability:
$$m^d = (a\cdot b)^d = a^db^d \mod N$$

Now regarding the possibility to "cancel out a factor" thanks to this malleability, it is partly possible and is actually used in the so-called "blind RSA signatures". (Read it, it explains all the maths.)
However it requires to be able to get the signer to sign a specially crafted message.