If you intend to link two ION speakers together of differing models, it would be wise to first check the ION Stereo Link Compatibility Chart, to ensure that the models you are attempting to pair are compatible with each other.
Assuming that your speakers are compatible for wireless linking, please refer to the Quick Start Guide of your particular ION speaker for pairing instructions.
If the two speakers that you are attempting to link are not listed as being compatible with each other, then you will need to do so using an XLR cable from one of the speaker's Mix Out, to the receiving speaker's Aux Input.