Signing 2x128 bits key vs 1x256 Is signing something 2 times with two different 128 bit keys as safe
as signing once with 256 bit key ?
Does signing 2 times with 256 bit key will make it stronger than
signing once ?
Ok, first things first: Your question actually touches on two subjects:


Is it better / as good to sign something with two different half-length keys than with one full-length key?
Is it better / as good to use two half-entropy passwords as one full-entropy password?


The (theoretical) answer to both of these questions is a clear no. However, details matter, which can turn the "no" into a "yes".

To adress the first question: If "half-length" means that each key has a secure keylength to begin with, then yes, this is about as secure as using a full-length key, because each key is unbreakable to begin with, meaning we have "unbreakable" vs "even more unbreakable" at which point other considerations should pitch in. For example it might be easier to ensure correct usage if two keys are forced. It will be harder to extract two keys using side-channels or malware than it will be to extract one.

However, if half-length means that only the full-length key has a secure standard key-length, then this will be completely broken. As an example consider Elliptic Curve keys (such as used by ECDSA in Bitcoin): One 256-bit key is unbreakable right now because you need $2^{128}$ operations to break it, but two 128-bit keys are very much breakable because they require less than $2^{64}+2^{64}=2^{65}$ operations to be broken (because you can re-use parts of the computations).

Now for the specific instance noted in the comments: 


  Are you familiar with bitcoin bip39 wallet creation ? My initial
  concern is, if multi-signature wallet created by two accounts with
  both seed of 12 words (128 bits of entropy) is safer or safe same as
  regular single signature account with seed of 24 words (256 bits of
  entropy ) ?


Now this is the question 2) I named above. While yes, this password will only offer 129 bits of security, this is already "unbreakable" at which point we would be discussing about "unbreakable" vs "unbreakable". Now nobody would successfully guess a 128-bit entropy secret, so it has to be stolen. It is harder to steal two secrets (which preferably are at different locations) than it is to steal one secret, so using two keys is actually preferable in this scenario.




  So it would be actually better to set up this account with 2x 24 word
  seed account which will produce 257 bits of [security].


Yes, it would be "better" in a theoretical sense. In a practical sense there's no security gain and all you achieve is making your own life harder by having to remember / type more words. Also see this Q&amp;A on why really long passwords aren't better.


  Out of curiosity, does ed25519 support full security of 256 bits ?


No, Ed25519 has about 126 bits of security. The linked source talks about Curve25519 but can be equally applied to Ed25519 with minimal transformations.


  at some point i've heard that bitcoin developers were saying that it's
  way more secure than Secp256k1 ecdsa


Theoretically no, practically yes. secp256k1 provides the "full" 128-bit security that you would expect and thus is "more secure" than Ed25519. However, Ed25519 is a more modern mode and curve than ECDSA with secp256k1 meaning it is easier to get right and secure against implementation mistakes. This (and the speed bonus) is why it's usually preferred over ECDSA.


  Also - is there any asymmetrical cryptography algorithm with 256 bit
  security, if yes - then why Bitcoin or other crpyotcurrencies are not
  utilising it ?


Yes, if you replace secp256k1 with the P-521 curve, you get about 256-bits of security. However this will also be a lot slower because there are likely less optimizations and the operations are more complex due to larger values. Also see this answer for some discussion on curve sizes larger than 256-bit.



One final note: When I'm talking about "x-bit security" here, I mean that $2^x$ operations are needed to carry out a successful attack. The actual keying material may be bigger than the "security bitlength" which is only the lower bound.