Ultimate magus is indeed the only arcane/arcane dual-spellcasting-progression class.1
You could maybe abuse legacy champion from Weapons of Legacy or uncanny trickster from Complete Scoundrel. These classes progress any one other class at some of their levels—which means that, unlike typical spell-progressing prestige classes, you could choose ultimate magus for them. The argument is basically that nothing says you can’t pick something you have finished, and the wording on them has it improves your level so level-scaling things are improved. The typical usage is hellfire warlock from Fiendish Codex II. The entire thing is on incredibly thin ice, as far as the rules go, and I wouldn’t expect many DMs to actually go for it.
Note that neither class offers progression at 1st, that uncanny trickster is only three levels long, and that legacy champion is rather painful to enter thanks to the terrible rules for legacy weapons, so even if this is allowed, you’re missing out on another level of your “primary” class, which is brutal, and either getting just two levels of your secondary class out of it or else have to deal with the awfulness of Weapons of Legacy.
Really, I don’t recommend it. Just accept that you traded one level of your primary spellcasting for eight levels of your secondary spellcasting and some useful class features (you did make sure to advance your primary class at every ultimate magus level, right?), and after 15th focus on the primary spellcasting with whatever other prestige class catches your fancy.
If that’s really not acceptable to you—and, to be fair, it is, perhaps absurdly, not an equitable trade, you do give up more than you get—then you are better off just saying that to the DM and asking for some kind of houserule extending ultimate magus or something. After all, you would definitely have to ask your DM about how legacy champion or uncanny trickster interact with “finished” prestige classes anyway, so while you’re there you might as well ask for what you actually want.
The other alternative, in a high-level game, is to use ultimate magus to progress some other prestige class that gets its own spellcasting—sublime chord from Complete Arcane or Nar demonbinder from Unapproachable East are the typical choices. Sublime chord is only ten levels long, so getting eight levels of progression gets you almost everything (and uncanny trickster could get you the other two, if desired), and Nar demonbinder is only seven so you finish it before you even finish ultimate magus. Note that neither sublime chord nor Nar demonbinder is Intelligence-based, so that is a downside (I’m fairly sure there is no strong Intelligence-based choice here—assassin is an interesting idea, but not a very good one). Sublime chord is also going to be very difficult to enter without losing more levels off your primary progression; I’m not 100% sure it’s even possible.
- Eldritch theurge from Complete Mage and noctumancer from Tome of Magic are both sort of arcane/arcane, but neither helps you—along with arcane spellcasting, the former progresses eldritch invocations and the latter shadow magic mysteries. These things are described as “arcane” but neither can continue to progress the same classes that ultimate magus progressed.