How about Edinburgh / Glasgow - Aberdeen calling all stations between Aberdeen and Dundee with alternate every other hour between Edinburgh / Glasgow and an express to Edinburgh and Glasgow splitting at Dundee?
Joining and splitting of trains is best avoided, the only practical use is at Aberdeen
Perhaps this will make it clearer (example) :
(xx40 Glasgow) xx00 Dundee - all stops - Arbroath - Montrose - Stonehaven - (Portlethen every second service) - Aberdeen
(xx20 Edinburgh) xx20 Dundee - Arbroath - Stonehaven - Aberdeen
(xx10 Glasgow) - xx40 Dundee - Montrose - Stonehaven - Aberdeen
(EC/XC) xx45 Dundee - Arbroath - Montrose - Stonehaven - Aberdeen
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I was presuming that the proposed hourly service that the OP was talking about would replace the current Glasgow - Dundee "shorts" (as well as the Glasgow - Perth bit of the Inverness services?) to make Glasgow - Dundee twice an hour so there would be the current service at Dundee each hour
That's the plan, there just isn't enough rolling stock just now, as discussed before
Once the Class 170 units are released from the E&G route they will be used on :
Edinburgh - Inverness (hourly)
Edinburgh - Aberdeen (hourly)
Glasgow - Aberdeen (two per hour)
However, this final proposal requires Network Rail to improve signalling between Arbroath and Montrose, parts of which have already been completed
The remaining Class 170 units will be displaced the GSW
In the short term Class 170 units will operate :
Perth - Inverness (every two hours, providing an hourly service)
Edinburgh - Perth (hourly, providing a half hourly service)
It is all a bit disjointed, but it is a short term solution to the increasing demand