I had FUT 2 years ago to reduce a high hairline, unhappy with results and want to increase density. Should I go for FUT/FUE?

It would be in your interest to have a consultation with a doctor and understand your risks and benefits of each surgery.  It would not be fair to suggest one or the other option without an examination or understanding your specific goals. There are issues of scarring, shock hair loss, hairline shape, and many other factors that must be understood.
For instance, FUE grafts are more fragile than FUSS (strip) grafts. FUE grafts are very prone to desiccation (drying up) and dying if not hydrated and implanted with care.
YOUR RESULTS ultimately will depend on all the factors as well as the doctor's aesthetic and artistic ability in creating a natural hair line.

From the photos, I would expect the addition of 1500 grafts (FUE or FUT) would be sufficient to fill in the hairline. The decision to go FUE or FUT is a personal decision for you to make, getting 1500 grafts through FUE should be easily disguised with the length of your hair.

FUT and FUE can both give excellent results in the hand of an experienced surgeon. You don't need to shave the hair in the recipient area and even in the back for FUE we can do layer shaving or even long hair FUE. Additionally no shaving is required for donor site for FUT either except for the strip itself.

I did not see how many grafts you had the first time.  The need for more density could be from poor growth or just not enough grafts transplanted the first time.  Not knowing what expectations you and the surgeon discussed there is more info needed.  
FUT is a great method for someone with long hair.  
Just make sure your expectations match what the surgeon is saying.

Hi. Thanks for posting your question with photos. I would strongly encourage you to have second procedure to fill in the gaps. Truly either FUT or FUE will work, so long as they are performed correctly.  In your case, I would lean toward FUT if you have scalp laxity as you already have an incision line. FUT avoids needing to shave a significant volume of your pony tail hair.  In the end, you should move forward regardless of technique.I have done numerous second transplants for women in your situation.  The results have been outstanding. I wish you the best of luck!

I think that you are focusing on the wrong issue. You said you were disappointed with the results of your last surgery and if some of the grafts failed to grow, it was most likely a problem with the surgical team (including the surgeon who is captain of the ship). Both FUE and FUT in the hands of experienced and well disciplined surgeons and teams, work reliably. From the pictures, I am assuming that you are female, so an FUT is a better option for two reasons (1) you don't have to shave the back of your head and (2) your had on FUT before.  This time get the right surgeon to do the right job, with FUT or if you are willing to shave the back of your head FUE.