Female Rectangular Headers with Long Pins (Pass-through style)
Generally there is no modification necessary. Two of the 1x8 headers can be used side by side to mimic the 2x8 configuration. (Note that: Yes it is possible to find long tailed dual row headers in the market).

You could probably use a small amount of epoxy or silicon adhesive to glue the bodies of the two single-in-line headers together to create the 2x8 equivalent parts. 
If you do this please be aware of some things to consider:


Use care to not get any of the glue on the pins of into the socket
holes.
The body thickness of single-in-line headers is usually a few mils
thinner than the nominal 100 mil row to row spacing of a dual in
line header. As such when you glue this can space the two parts out
to 100 mils center-to-center if the correct thickness of the glue
layer is maintained.
Some header body sides are not 100% parallel. This is due to the
molds used to make the parts where the cavity walls are tapered ever
so slightly to allow the parts to easily exit the mold. Depending
upon the particular SIP connector headers you have in hand this mold
release angle could be more or less apparent. When the two bodies
are sandwiched side by side this can tend to cause the long pins in
the two rows to be non parallel and one row can tip toward or away
from the other.


The last two items above may be inconsequential considering the available tolerances of pin headers and the ability of the pins to flex around in their housing pockets.