I think it is as much an SRG issue as anything else.
They've been against secondary validations for a long time. I used to work for Highlands and Islands in Haggisland and held dual validations, in theory for the purpose of leave relief, but in practice it was because of the major recruitment problem that they had.
Great for the extra dosh and air miles, but it does nothing for the brain when you finish on day 3 at one unit and go to work at the other unit on day 4.
As a result of this, I ended up making a simple - but potentially very serious mistake - because I applied one set of procedures for airfield A whilst working at airfield B.
Due to the lack of any positive action by HIA, I filed a report with CHIRP, the end product being HIA being heavily leaned upon by SRG, and the end of duel validations in all but the most desperate of cases.
In the vast majority of cases it would now be very difficult for any atco to take secondary atco employment due to SCRATCOH, but you can, as many do, go and do an unregulated number of hours at a FISO unit, and get well paid for doing so.
An atco could also do as much flying as they wished, regardless of how dim a view SRG took of it, the only responsibility the atco has is to ensure that they are fit for duty.
Additionally, SRG could never regulate secondary employment outside the atc sphere.