How to display multiple subtitles concurrently with MP4 or 3GPP

[diro]

I am trying to dislay two subtitles concurrently with an 3GPP file(.3gpp) by QuickTime and OSOM4, but only one text track showed.
Does anyone know how to display two or more subtitles concurrently?
Thanks.

[bond]

i guess (!) this can be done by using different layers, but i never tried it

layer : defines text stream z-order (type: signed short). This is only needed when composing several text streams in a single presentation: more negative layer values are towards the viewer.

[diro]

Thanks, bond.
I had tried to use layer but failed. The QuickTime won't display any subtitle,and OSOM4 will let user to choose subtitle1 or subtitle2.

Follow is my subtitle tracks information:

Track # 1 Info - TrackID 1 - TimeScale 1000 - Duration 00:00:13.996
Media Info: Language "und" - Type "text" - Sub Type "tx3g" - 4 samples
3GPP/MPEG-4 Timed Text - Size 620 x 64 - Translation X=10 Y=10 - Layer -1

Track # 2 Info - TrackID 2 - TimeScale 1000 - Duration 00:00:13.996
Media Info: Language "und" - Type "text" - Sub Type "tx3g" - 4 samples
3GPP/MPEG-4 Timed Text - Size 620 x 64 - Translation X=10 Y=300 - Layer -2

[diro]

I know what is the problem now..
I create this .3gp file by MP4Box:

mp4box -add rawdata_1_text.ttxt -add rawdata_2_text.ttxt -add movie-av.3gp 2sub.3gp

and the ftyp box will have a major brand '3gg6' , after I modify it to '3gp6' , it will display the two subtitle tracks concurrently.

3gg6: 3GPP Release 6 General Profile
3gp6: 3GPP Release 6 basic Profile

[bond]

I know what is the problem now..
I create this .3gp file by MP4Box:

mp4box -add rawdata_1_text.ttxt -add rawdata_2_text.ttxt -add movie-av.3gp 2sub.3gp

and the ftyp box will have a major brand '3gg6' , after I modify it to '3gp6' , it will display the two subtitle tracks concurrently.

3gg6: 3GPP Release 6 General Profile
3gp6: 3GPP Release 6 basic Profilein quicktime? you might want to tell the gpac devs about that