You may benefit from watching the essentials videos they are a great source of information and will give you the basics of node-red and the uses for the standard nodes. There is also the cookbook with many examples of using the standard nodes. Take a look at Flow conrtol and Create a single message from separate streams of messages, which is an example of joining messages.
[edit] Or wire the modbus nodes in series and move each msg.payload to a new message property betwenn reads. e.g Modbus Serial - Whole bus disconnects when one device drops - #16 by Steve-Mcl
