There really isn't a good way to have multiple overlays in the AFNI GUI.

You could multiple the stats map DSET_STATS by the region map DSET_ROI, adn show that:

3dcalc -a DSET_STATS -b DSET_ROI -expr 'step(b)*a' -prefix DSET_STATS_IN_ROI

... and then use DSET_STATS_IN_ROI as the overlay.

A somewhat cheating method might be to concatenate the stats volume and the ROI one, and use the ROI one as the alpha map

+ Make an "elevated ROI" map, which is 2 in your ROI, and 1 elsewhere that you want to see stuff (and maybe 0 outside that); for example, here is one that is 2 in the ROI and 1 elsewhere:

3dcalc -a MMM1.nii.gz -expr 'step(a)+1' -prefix DSET_ROI_ELEVATED.nii.gz

+ Concatenate your desired stats volume and the 'elevated ROI map':

3dTcat -overwrite -prefix dset_stat_and_mask.nii.gz stats.FT+tlrc.'[2]' DSET_ROI_ELEVATED.nii.gz

View it with @chauffeur_afni now, thresholding the elevated ROI map at, say, 1.5:

@chauffeur_afni         -echo                                              \
    -ulay             MNI152_2009_template_SSW.nii.gz                 \
    -olay             dset_stat_and_mask.nii.gz                       \
    -box_focus_slices AMASK_FOCUS_ULAY                                \
    -func_range       3                                               \
    -cbar             Reds_and_Blues_Inv                              \
    -thr_olay         1.5                                             \
    -olay_alpha       Yes                                             \
    -olay_boxed       Yes                                             \
    -set_subbricks    0 0 1                                          \
    -opacity          6                                               \
    -prefix           IMG_ROI_STAT                                   \
    -set_xhairs       OFF                                             \
    -montx 3 -monty 3                                                 \
    -label_mode 1 -label_size 4

--pt