I think you want something like this script. Save the script and run it as suggested below. The key to this is the sub-brick list can be used to specify the order to load the various sub-bricks. By alternating the sub-bricks between the lower and upper ranges, 3dTcat will make a new alternated dataset.

#!/bin/tcsh

# mkalt.csh
# tcsh mkalt.csh dset split_sb
#
# tcsh mkalt.csh spiral_in_out 165
set dset = $1
set split_sb = $2
set outprefix = `@GetAfniPrefix $dset`

# generate an alternating sub-brick list
set sbl = "0,$split_sb"
foreach sbi (`count -digits 1 1 $split_sb`)
set sba = `ccalc -int "$sbi+$split_sb"`
set sbl = `echo $sbl,$sbi,$sba`
end

# now spit out the jumbled list
3dTcat -prefix ${outprefix}_alt ${dset}"[$sbl]"