If you decline the backup restore, it will only no longer be possible to restore it on the iPhone you are using, unless you choose to reinstall WhatsApp again. It is then safe to decline the backup restore on your old iPhone.
To avoid losing your current data from the backup, make sure to disable auto-backup on your old iPhone. Otherwise, your data will be overwritten by the new data of that iPhone.
Once you switch back to your main iPhone, your backup will still be available. All you have to do to is to uninstall and reinstall WhatsApp to restore your backup.