I think it depends ....i know the one birth i "doula'd" for a friend, scarred me for life...I will die with the hatred for that OB in my heart, and given a "freebie" from God, i would kill her with my bare hands and feel no remorse.......I could NOT do that again, ever, but certainly not while pregnant.....That's the emotional side...on the physical side..i would think you would set a limit for yourself...doula'ing is not only hard emotionally, but physically...what if you get a mom with a 48 hour labor who needs massaged the whole time ? You have to look at your ability to physically do the job...for me, there would be a cutoff point...
Also, there are no guarantees about your physical state...itsa great your first pregnancy went smoothly..but you could end up with raging morning sickness this time...or a complication..so i think you would just need to know that and be flexible going into it....
Personally, i would be possibly hesitant about hiring a pregnant doula....depending on how far along you were...my midwife with dd, was pregnant and due about 3 months before dd...she was out that month(my 7th month), but then came back to work...when i was 8 months...it worked out fine...
anyway, those are just thoughts i have.......I think you could easily do it, if things work out......but if it is something you depoend on, for income or something, i would be hesitant to assume you are "definitely going to be able to", you know what i mean?