NT quota is roughly the same as Tas, so doubling the territories would mean in the NT you would only need 12,000 votes vs NSW 260,000. ACT would also drop to 38,000.
Only way to get change via a referendum is to keep Qld and WA on side. Currently they are 160,000 and 85,000 respectively so you would need to increase the representation of WA in the senate as the minimum and this will be difficult if NSW and Vic are going to nearly double their representation.
Agree its a problem that cannot be easily fixed and unfortunately the extra seats generated in the 80's made the situation worse. Tas and SA population growth is just above near flat line and not likely to change in a hurry. With overall population growth the lower and upper house numbers will continue to grow, however I would not expect either major party to support or promote growth of the Senate. However if this does happen, Tas representation will become increasingly skewed and ridiculously small.
I'm sure at some point in the distant future there will be some very hard questions asked why NSW is pushing 500,000 and Tas barely 10,000!