So, can stack-based programming languages be concurrent?
Sure.
Could they achieve concurrency by using multiple stacks at the same time or something alike?
Already for normal languages multi-threading usually means having multiple "call" stacks. It would be completely natural to give each thread its own data stack. It would be straightforward to have an actor, say, whose body was implemented by code in a stack-based language. Explicit parallelism a la GHC's par annotations should be reasonably straightforward. The main issue with executing things in parallel is that you don't know what the stack effect of the code will be until you execute it. However, using a Joy-like syntax, one could imagine [a b c] par as executing a b c either against an empty stack or a copy of the stack and only keeping the top-most element of the stack on completion (or pushing some dummy value if the stack is empty). You could imagine variations. Implicit parallelism would be harder to do naively as compared to, say, a purely functional language, but certainly could be done as well. The compiled code of a user-defined combinator is often not too different from "normal" code.
Is it possible to implement lazy evaluation in a stack-based programming language?
Unknown stack effects are again the tricky part. If you design the language such that all stack effects can be determined statically, then it doesn't seem too challenging. If you have laziness be explicit, then it also seems relatively straightforward and would look roughly like Joy's quotations and i. One language that calls itself a lazy concatenative language seems to do a mixture of both of the above approaches from what I can tell. I don't really see how an implicitly lazy concatenative language would work in the face of dynamic stack effects, at least not in any vaguely usable way, but that might just be a lack of imagination on my part.
Incidentally, it is not uncommon for stack-based languages to have multiple stacks, e.g. Forth has a data stack and a return stack upon which you can also place arbitrary data.
