Base64 doesn't have a concept of "stream". Conceptually base64-encoded string with padding is a concatenation of fragments that are always 4 bytes long but can encode one to three bytes. Concatenating two base64-encoded strings with padding therefore don't destroy fragment structures and can be decoded into a byte sequence that is a concatenation of two original input sequences. Without padding, fragments can be also 2 or 3 bytes and short fragments are not distinguishable from long fragments, so the concatenation will destroy fragment structures.

It would decode correctly but you wouldn't know the boundary, if that matters. I see, thanks.