That's just not true - that's one of the typical "myths" banded around by people.
I run twin 1.5" SU's on one of my cars and have no problems with them "going out of tune" - I've never had to fiddle with them to retune once they are setup. I've been using them on the same engine for over 9 years. I have two wideband sensors installed in that car and I know 100% what the mixture should be and would definitely notice any "going out of tune".
Get yourself a synchrometer and you can set them up yourself (assuming they already have the correct and identical needles and all you want to do is balance them).
You have to take your time, slacken the linkages off, adjust each one till their sucking the same then tighten the linkages back up and check both are engaging at exactly the same time. You need to set something the same on both to start with - let's say the jet height at somewhere sensible and make it the same both sides. Then balance the airflow on both carbs, keep adjusting the idle speeds and readjusting the mixture screws to allow for more/less airflow. You'll probably find you can close the butterflies a fair bit from when you started by constant retrimming of the mixture to suit - if you have a vac gauge you'd see the engine pulling more vac at idle than when you started the tuning/balancing.
I find the large bore A series engines idle best around 12.5-12.8:1 if they've got anything hotter than a 276 cam in - anything like stoichometric is a no no! They can obviously run much leaner once on the move/cruise etc
It's not difficult - just take your time.
If you need to start custom profiling your own needles then I agree to a point with DB - twins become more tricky as getting two very close needles takes time. Been there and done that more times than I care to recall
Do you have a wideband in any of the manifold branches?