Some versions of SVR4 (Esix and Microport) have mechanisms for enabling
two-key sequences for VT switching (Alt-Fn).  
The standard SVR4 mechanism
is Alt-SysReq-Fn, which all versions we know use.  
Running under X, the
Alt-Fn sequences are stolen by the driver before the 
server can see them,
so you can't use them for X applications.  So you want to switch back to
the standard 3-key sequences while you are running X.  Here's how to do
it:
Microport makes this very simple.  The 2-key mode is called
"Microport Mode", and the 3-key mode is called "Compatible Mode".
You enter Microport Mode by pressing Alt-SysReq-m.  You enter
Compatible Mode by pressing Alt-SysReq-c.  So all you 
need to do is press Alt-SysReq-c after starting the
X server to allow X clients access to the
Alt-Fn sequences.
Esix has no keyboard-driven way to switch modes. There are two levels at which this can be handled:
/etc/conf/pack.d/kd/space.c.  When set to 1 (the default), 
2-key
mode is enabled.  When set to 0 it is disabled.
xc/programs/Xserver/hw/xfree86/etc/
in the source tree, and in 
/usr/X11R6/lib/X11/etc/ in the binary
kit).  You can compile and install this program.  Then to make use
of it, add the line `VTInit "2key off"' 
to the Keyboard section of
your XF86Config file to cause the program to be run 
automatically
when the server starts up.  Doing this means that 2-key switching
will be turned off while in the server's VT, but will still be on
for the other VTs.