The "upsell" type of offer (replace one product / variant with another) only supports a single offer.
For cross sell offers:
1. Open the "funnels" tab.
2. Click to add a new funnel or edit an existing one.
3. Scroll down to the offers section (the one with the purple background). You'll notice each offer has a number to indicate its order ("Offer #1").
4. Scroll down to the bottom of this section, and click "Add another offer".
5. A new offer (marked as "Offer #2") has been added and will be shown to your buyer right after "Offer #1".
6. Select the product you want to upsell, discount type and set a custom offer text.
7. Click "Save and Activate"