that's more insurance!
Assuming the shower water pipe came UP under the side wall shower heads, water would dribble out through one or both shower heads. But, if the pipe came across the attic over to the shower, then putting a side wall shower head under the ceiling mounted one would not give much insurance of draining the pipe run crossing the attic.
Geofixit, you better have some serious eyeballs look at your space.
Ultimately nothing guarantees there will be no freezing. Not even a lot of insulation, in a large airtight (closed-to-air) enclosure. Insulation slows down the transfer of heat and cold. It does not "keep things warm".
The longer you go without running water in the shower pipe, the colder the stagnant pipe water gets, second by second. Then, at some temperature point, things stabilize assuming the indoor and outdoor temperatures are stable in this example.
When the heat transfer from the heated part of the house into your shower pipe equals the heat transfer from the shower pipe out to the great outdoors, the water in the pipe has reached its stable temperature. Whether the temperature in the pipe reaches a steady-state temperature above or below freezing is not knowable in advance. Too many variables. How well you execute your insulating and sealing is a major factor.
The colder the outdoor temperature, the greater the heat loss. Duh. When the temperature spread (between room temp and outdoor temp) doubles, heat loss is quadruple as fast. Ah ha.
David