Is there support for using both x.25 socket and tcp/ip
socket in the same process?
Yes, provided the kernel supports both protocol families. As far as
I'm aware, only BSD4.4 systems (and those derived from it) support
X.25 and TCP/IP sockets.
In other words, can system call such as select, read and write work
with both types of protocol?
Yes, though some things such as the handling of out of band data will
Does Solaris 2.4 support such scenario?
No. AFAIK, this has no support for X.25 though it may be available as
an unbundled option. BTW, Solaris doesn't "support" sockets. The
networking subsystem is STREAMS based, and has an sockets emulation
library for backwards compatibility.