Wow, If that's all you want to do is jump between two different programs at a fixed interval then I would just start a timer interrupt at your favorite interval then TOGGLE a BIT called "PROG" each time the interrupt occures as well as set a flag bit called "Interrupt" to indicate the interrupt has occured. Then your main program would look like:
MAIN:
if Interrupt = 1 then
Interrupt = 0
if PROG = 0 then
gosub program 1
else
gosub program 2
endif
endif
goto main