Yes.
In some cases, with electronic ballasts, I notice adding the second bulb
causes the first bulb to dim just a bit as the ballast gets loaded. I can't
say if that the second bulb is causing the current to come down to rated
current or fall below. At any rate, ballast factor must be addressed when
comparing lumens.
There are other effects too. For example, the positioning of
the second lamp is likely to create a shadow of the first
lamp in most practical scenarios, i.e. some of the light from
the first lamp enters the second lamp. Whilst a little of this
might be reflected/refracted out again, much of it will be
absorbed and lost, resulting in output which is less than the
sum of the two by themselves. This is actually an issue for any
type of folded or bent tube too, as used in compact fluorescents,
where much of the light can be lost into other parts of the tube.
In most designs, when you look at the tube, there is a significant
portion of the tube you can't see because it's obscured behind
another tube section, i.e. you're in its shadow. It's pretty much
only linear tubes which don't have this issue.
