Hi, Derek,

DC> `*IF* the gifts *OF THOSE WHOSE* requisites which we use - the robes,
DC> alms-bowl, chair, bed, and medicine in support of sickness - will
DC> have great fruits, great merits [for the people who give them], and
DC> *IF* our going forth will not be in vain, will be fruitful, will have
DC> a result'

There's only one 'if':

yesañca maya.m paribhuñjaama
ciivarapi.n.dapaatasenaasanagilaanappaccayabhesajjaparikkhaara.m
tesa.m te kaaraa mahapphalaa bhavissanti mahaanisa.msaa,
amhaakañcevaaya.m pabbajjaa avañjhaa bhavissati saphalaa saudrayaa

Literally:

Whose necessities (robe, alms-bowl, seat & bed,
medicine as help in illness) we use,

deeds of those

will be of great fruit and great merit,

if this our going forth won't be futile and will bear [lit. be of]
fruit and result.

Hey, I'm starting to understand it myself ;)

Dimitry