Is the cause of Old Faithful's bimodal distribution for eruption times straightforward?
First I would like to say this is not my area of expertise, so this should perhaps be a comment but I am not allowed yet to comment on this site.

In a simplistic view a geyser has heated water chamber with a column of water above it.  The water is heated above the normal boiling temperature but is kept liquid by the pressure of the water column above it.  When the water eventually starts to vaporize this reduces the pressure on the water in the chamber resulting in more rapid vaporization of the water in the chamber resulting in the eruption.

For Old Faithful assume two chambers, one chamber is lower than the other. If the temperatures in both chambers are high enough the vaporization in the upper chamber would also reduce the pressure in the lower chamber allowing it to vaporize resulting in both chambers emptying, resulting in a longer eruption.  Assuming the infill and heating rates are essentially constant then the time before the next eruption would take longer because both chambers are "empty".  On the other hand if the upper chamber vaporizes and but the lower one does not, either because it is not hot enough or the pressure hasn't dropped enough or back pressure from the upper chamber keeps the lower chamber liquid, then only the upper chamber will empty resulting in less time to infill and reheat.

Again not my area.