There has been so much discussion on what is required for an upgrade over the years, that I have been keeping track of my own experiences for reference. My experiences over the last two years has been simply positive J availability, either on the segment (if only upgrading 1 leg) or on the married journey if upgrading the entire route with 1 voucher. Here are my data points for successful upgrades of 2 pax (using each person is E+ on different PNRs). The J-space below was based on EF availability data before booking... of course, YMMV:

Route Married / Individual
YYZ-IST-MRU J2 / J0 & J4
YYZ-IST-CAI J0 / J4 & J0 (only upgraded YYZ-IST)
JFK-IST-KTM J4 / J3 & J4
BRU-IST-CAI J1 / J4 & J1 (upgrade for the complete journey was available for 1 pax; we chose to upgrade only BRU-IST for 2 pax)

In each case above, the availability to upgrade by miles on the web-site was the same as for the E+ voucher.

On a side note, my last two interactions with TK have been shockingly fantastic! The E+ upgrade process was completed without error, the first time, in less than 5 minutes! This is not to say that I haven't had the same horror stories as others in the past.