There is not a techincal reason this can't be done and I have seen people do it. Probably not the best because we recommend the sub to be the primary callmanager but you really don't want to do that when it is 7825. And you really want the 7835 to be the pub since pub is doing all the database stuff.
We were just about to do the same thing. We currently have two 7825's working fine - no load problems, etc (only 150 phones total :-) and were going to add a new 7835-1266 as the publisher basically only for the reason of having a supported way to mirror the publisher drives to ease our minds on call manager upgrades - if we're satisfied with the performance of the two 7825's now - should there be a reason to worry once neither has to do the publishing duties and both can split the primary/subscriber duties?
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.