Actually you can do nearly anything that can generate a true/false condition.. You often use () to set up which goes with which comparison..

Try this:

IF NOT EXISTS ( SELECT 'X' FROM dbo.SomeTable WHERE SomeField = SomeValue )

AND NOT EXISTS ( SELECT 'X' FROM dbo.SomeOtherTable WHERE SomeField = SomeValue )

BEGIN

-- Do some work here that you want to do

END

Its a little pseudo codish but I think it shows what you are asking for..

Or you can mix it up some..

IF NOT EXISTS ( SELECT 'X' FROM dbo.SomeTable WHERE SomeField = SomeValue )

AND @SomeVariable IS NOT NULL

BEGIN

-- Do some work here that you want to do

END

CEWII