What is the value of a 1994 US cent with part of the obverse stamped on the Lincoln head side and attached to a 1995 penny?

For proper evaluation, an error coin should be examined in person by a dealer or appraiser who works with errors.

The best that can be said at this point is that it's unlikely (but not impossible) for your coin to be a Mint error as it has two separate dates. It could be that someone hammered two coins together after they were minted, or somehow a 1994 cent found its way into a coin press in 1995 and the two were mangled together. Again, anything more definite would require in-person inspection