There are many factors that need to be taken into account. Not Knowing the details of your case, the manner in wich the teeth were lost, the lenght of time since the teeth were lost, the gingival type,the amount and anatomy of the residual/existing bone,the type of implants you use and many more things need to be assessed and evaluated.
I would first ensure there is or reconstruct enough soft and hard tissue; if by any chance #7 and #10 need crowns, then I would graft and give the patient a long term temporary bridge with ovate pontics at #8 and #9. Later at approximately 4-6 months I would place two implants (in my case most likely Astra or Hiossen)and either have custom healing abutments or immediately temporize them in order to preserve/enhance ginigival contours. In some instances a second soft tissue graft might be necessary.
The option of cantilevering should also work,never tried it personally. The worse prognosis for papilla maintenance/regeneration is between two implants and with this option you definitely minimize your risks.

In this scenario of replacing #8 and 9 what local parameter do you look at in deciding to place two vs 1 implant and cantilever?

There must be some situation where you look at a case and you say definitely one implant only?

I have to assume that you mean by "local parameters" the clinical and radiographic information the patient presents with in order to decide cantilever or two implants at #8 and #9.
1)I would first assess the patient expectations, smile line, occlusion, gingival type, the size of the space; because more often than not most patients I have seen that with such cases have lost the teeth for a while and as a result there is a strong possibility of tooth movement and bone loss that even in ideal conditions two implants might only be a part of the solution for good results,you might even have to address the adjacent teeth (crown/veneer).
I would also look at the mesial bone levels of #7 and #10 because once you raise that flap and healing occurs, tissue shrinkage might pose a challenge for you at restoration time.
2) I obtain a CAT Scan of all my patients in order to determine the quantity and quality of bone. The treatment planning software I use is InVivo from Anatomage and some of their rendering features allows me to evaluate the height of soft tissue above the residual bone, this is particularly useful when you try to determine how much vertical height you have or might need to reestablish.
The scan also allows me to plan implant placement, make more detailed space analysis and order a CAD generated surgical guide.
3)For such cases I have drifted away from immediate placement, I prefer site preparation with adequate temporization. Implants are then placed later in better bone and soft tissue environment. Custom Made esthetic healing abutments are placed on the implants during integration time.
4) Based on some "local parameters" implants might not even be an option, then look at a conventional bridge with ovate pontics sometimes back that up with a Connective tissue graft where needed.

In closing it squarely comes down to soft tissue and supporting bone especially in the areas of #8 and #9 where aesthetics are a primary concern.

Tarnow lectures all over the place on placing 1 implant and cantilever. What is your thoughts on tarnows study and his reteric on 1 implant and cantilever in the esthetic zone?

Depends on how much space is present. You need 3mm between implant fixture (minimum) and 2mm between the implant and natural tooth, so if you going with 4mm fixtures you need 2+4+3+4+2=15mm to place two fixtures. or using 5mm fixtures 17mm.