It's nice to treat your other half in this way..., so check this out:
1/ Companion Ticket:
Revenue J Tkt (I): HK$46,808 (taxes included)
Companion Tkt Redemption Asia Miles required: AM75,000 + Taxes
2/ Y+ Ticket x2 & Up-grade redemption: (updated)
Revenue Y+ Tkt (W): HK$34,698 x2 = HK$69,369 (taxes included)
Up-grade Y+ to J Redemption Asia Miles required: 30,000 x2 = AM60,000
(Taxes will be increased due to premium class to LHR

)
3/ J Ticket Redemption & Y+ Ticket Up-grade:
Revenue Y+ Tkt (W): HK$34,698 (taxes included)
Up-grade Y+ to J Redemption Asia Miles required: AM30,000
(Taxes will be increased due to premium class to LHR

)
J Ticket Redemption Asia Miles required: AM120,000 + Taxes
Option 3 seems the best. Good luck!