To add to this: if I remember correctly, it was relatively common for (passenger-equipped) steam 'helpers' to have steam lines connected to augment diesel SG capability in severe conditions or cold weather. I would doubt there are many examples of SG-equipped diesel helpers augmenting steam from the head end, but I wouldn't be surprised to find examples, ATSF perhaps being one.
Almost all SGs are set up for automatic firing based on pressure, and it would not imho be difficult to arrange this so that only if the steam engine (road or otherwise) wasn't putting out sufficient steam, whatever number of SGs would come on and cut off depending on fairly immediate demand. The developed steam all would go into the same trainline, without cutoff or stop valves that had to be opened and closed frequently as the train ran.
An interesting question, that I should know an answer to but don't, is how many streamlined A units were equipped with nose steamline connections. Naturally most if not all sets of A-B-A locomotives would be set up to supply a consist in either 'bidirection', but there might be some purpose-built sets (in the age when it was still politically 'preferable' to call a consist of units 'one locomotive' for labor-negotiation purposes) that were intentionally turned so one cab was always leading.