There are two possible options with multiple monitors support in RDP Client.
- Span mode.
First option allows you to view multiple remote monitors on single client monitor, on the other hand, multimon feature allows you to view remote desktop on all local monitors.
In your case you need first option. I also recommend reading this article for more in-depth understanding of requirements and limitations in both modes.
Remote Desktop Connection Usage
[<connection file>] [/v:<server[:port]>] [/admin] [/f[ullscreen]] [/w:<width> /h:<height>] [/public] | [/span] [/multimon] [/migrate] [/edit "connection file"]
/span -- Matches the remote desktop width and height with the local virtual desktop,
spanning across multiple monitors, if necessary. To span across monitors, the monitors must be arranged to form a rectangle.
/multimon -- Configures the remote desktop session monitor layout to be identical to the current client-side configuration.
You can also use Teamviewer which works well with multiple monitors.