Hi Juanito,
after all the freerdp didn't work with the "/Multimon:force" option
xfreerdp /sec:rdp /drive:USB-Stick,/mnt/ /multimon:force +multitransport +auto-reconnect -themes -wallpaper /cert-ignore /bpp:15 /kbd:0x00000407 /d:domain /v:server:port /f
[10:31:04:365] [5420:5421] [INFO][com.freerdp.core] - freerdp_connect:freerdp_set_last_error_ex resetting error state
[10:31:04:365] [5420:5421] [INFO][com.freerdp.client.common.cmdline] - loading channelEx rdpdr
[10:31:04:365] [5420:5421] [INFO][com.freerdp.client.common.cmdline] - loading channelEx rdpsnd
[10:31:04:365] [5420:5421] [INFO][com.freerdp.client.common.cmdline] - loading channelEx cliprdr
[10:31:04:787] [5420:5421] [INFO][com.freerdp.primitives] - primitives autodetect, using optimized
[10:31:04:795] [5420:5421] [INFO][com.freerdp.core] - freerdp_tcp_is_hostname_resolvable:freerdp_set_last_error_ex resetting error state
[10:31:04:796] [5420:5421] [INFO][com.freerdp.core] - freerdp_tcp_connect:freerdp_set_last_error_ex resetting error state
[10:31:04:807] [5420:5421] [ERROR][com.freerdp.core.gcc] - WARNING: true multi monitor support was not advertised by server!
[10:31:04:807] [5420:5421] [ERROR][com.freerdp.core.gcc] - Sending multi monitor information anyway (may break connectivity!)
[10:31:05:912] [5420:5421] [INFO][com.freerdp.gdi] - Local framebuffer format PIXEL_FORMAT_BGRX32
[10:31:05:912] [5420:5421] [INFO][com.freerdp.gdi] - Remote framebuffer format PIXEL_FORMAT_RGB16
[10:31:05:048] [5420:5421] [INFO][com.winpr.clipboard] - initialized POSIX local file subsystem
[10:31:05:146] [5420:5442] [INFO][com.freerdp.channels.rdpdr.client] - Loading device service drive [USB-Stick] (static)
[10:31:05:147] [5420:5421] [INFO][com.freerdp.channels.rdpsnd.client] - [static] Loaded fake backend for rdpsnd
it says that the multi monitor support was not advertised by server, but it actually works with TinyCore with the same freerdp version.