A demo for Tcl/Tk's `[send]` command. (Requires Tk8.6 on Unix with Xwindow support).

AppX.tcl

#!/usr/bin/wish
# -*- mode: tcl; coding: utf-8 -*-

tk appname AppX

set X 0

pack [button .b -text "Increment var in AppY" -command {
    send AppY [list incr Y]
}] -fill both -expand yes
pack [entry .e -textvariable X] -fill x -expand no

AppY.tcl

#!/usr/bin/wish
# -*- mode: tcl; coding: utf-8 -*-

tk appname AppY

set Y 0

pack [button .b -text "Increment var in AppX" -command {
    send AppX [list incr X]
}] -fill both -expand yes
pack [entry .e -textvariable Y] -fill x -expand no