Sometimes both appointments are made at once (i.e. before the first appointment). Sometimes you're expected to wait for the second appointment.
I was in the latter case. I had my first jab and was told I would be contacted to book my second one. After exactly 11 weeks I rang the GP and was given some lame excuse. I waited another three days, rang again and was given another lame excuse. I went online and booked my own second jab, at a centre much further away. After a nice little trip out, job done, both jabs administered.
Except now my GP is sending me text after text asking me to book my second jab!