the general convention fir use of Pantos is that trailing panto is used. If the rear one is damaged, the front one is used [1].
winter time during fog is where most of the front panto operation is seen, but i unfortunately couldn't find a source for this.
higher speed of train is seen more in rear panto design, but this seems to be an effect of confirmation bias (sort off)
can...
more... they be used alternatively?? well maybe, but the whole process of getting the other panto in operation involves switching off all the components in loco, lowering the panto, then raising the other one and going through the tedious process of starting everything again. not time consuming, but quite long. so that, while possible is unlikely to happen unless a LP has too much time to kill :P
[1] "General Description", MAINTENANCE HANDBOOK ON PANTOGRAPH AM-12 & SIMILAR DESIGN, CAMTECH