There is a great guide about dual ADSL/VDSL setup for the 2850. My new 2860n does not have the DSL port splitter in the box. So two questions:

(1) can the 2860 be setup to have ADSL failover from VDSL with the 2860 acting as "modem" for both and
(2) if so, how do I get one of the splitters?

No, it cannot. The Vigor2860 can operate on either ADSL or VDSL, not both at
the same time unless you add a Vigor120 to WAN2.

regarding adding a vigor 120 v2 (with latest firmware) to wan 2 of the 2860

Now you will be able to see some important stats (such snr margin) of the vigor 120 to the 2860
(currently works for 2860/2925)
http://www.forum.draytek.co.uk/viewtopic.php?f=3&t=20008#p82844
(that not works with any other brand manufacturer modem connected to wan 2 but only with the 120 v2)
so if anything get a 120 v2

EDIT

Or if the second line ever will be updated to VDSL (FTTC) as well, look for a vigor 130
The 130 can do both adsl & vdsl (the 120 only adsl)
& with the 130 will be able to see line stats (mainly if ever λινε issues to report at your ISP)