Hi
The old man, who I thought had fallen alseep, along with this other guy standing nearby, both laugh.
The old man = subject 1
who I thought had fallen alseep = modifies the subject 1
along with = conjunction
other guy standing nearby = modified subject 2
The old man, who I thought had fallen alseep, along with this other guy standing nearby, = compound subject, of which both are modified. This is the same as
Peter and Paul
Peter and paul (both) laugh -- OK
How about them:
The old man, who I thought had fallen alseep, along with this other guy, were standing nearby, both/0 laughing.
or
The old man, who I thought had fallen alseep, along with this other guy standing nearby, were both/0 laughing.