Safe to assume your rate of charge was ~half the norm one you plugged in?
No, only if you happen to be at a Urban Style Supercharger terminal is the capacity of the Supercharger evenly divided between two Teslas. So the latest Superchargers cabinets have a capacity of 145 kW and for Urban Supercharger terminals each gets up to 72.5 kW.
It is not so simple for the original style Superchargers. There is a dynamic algorithm that considers which car is charging first, and then starts allocating capacity (I believe in roughly 30 kW increments) between the two Teslas based on their relative states of charge. So if a Tesla with a low state of charge pulls into a terminal not being shared with another Tesla, then the capacity could be as high as 120 kW of the available 145 kW. If another Tesla pulls into a terminal that is being shared by the original Tesla, depending on the state of charge of the original Tesla then, it might lose about 30 kW of capacity. Over time, depending on the relative states of charge between the two Teslas, another 30 kW of capacity might be allocated to the second Tesla from the total available 145 kW.
This is an area where I believe Tesla could be doing better...directing drivers via the map UI as they approach a SC, which stall to utilize in order to ensure the most efficient use of the stalls relative to the status of each vehicle that is already at the stalls. The information is known or could be known at The Mothership, so it's a matter of bothering to program for it and push it out OTA. Doing this sort of thing, which not only improves customer sat but also increases the effective number of SC stalls with zero incremental capex (or continuing opex) dollars, feels like a no brainer to me.
Even if Tesla knows the instantaneous best stall for a newly arriving Tesla, it can't predict whether that stall will remain the optimum as other Tesla's arrive and leave the Supercharging Station. Ideally the objective should be to maximize the throughput of the station. So as cars pull in and out the situation radically changes such that the value of knowing the best stall when arriving may have little to do with minimizing your charge time during your stay at the station.
In the future, if Supercharger Stations become habitually congested, the best solution would probably be to tie all the Supercharger cabinets to one electrical bus and each terminal to that combined bus, instead of pairs of terminals to one Supercharger cabinet. Then with the appropriate new software on the station, it could automatically and instantaneously allocate capacity among all the cars at the station considering relative states of charge, and other parameters, to maximize the overall throughput of the station and the drivers would not have to do anything or consult a display to be assured of the best charging experience.
The above is my layman's simplified description of a method for Tesla to distribute power to multiple terminals at a Supercharger Station. After a Google search I found this Tesla patent application that appears to do essentially what I have described.
MULTIPORT VEHICLE DC CHARGING SYSTEM WITH VARIABLE POWER DISTRIBUTION
Larry