Silicone does not stick to plastic very well. Probably will be OK if you absolutely do not flex the pair of bottles, but the least disturbance and the silicone will pop off.
Here is an idea that you will have to research a bit.
My father made a toy for the grandchildren. It was 2 bottles of 2 liters each, connected at the opening. He partially filled the system with water and glitter. You would swirl the bottles and turn them upside down and watch the whirlpool as the water swirled from the upper bottle to the lower.
Here is my point: The system never leaked! There was some sort of tubing connecting the bottles. You might find it in a 'science for kids' sort of store, or a hobby shop. I have no idea even what to ask for. Good luck.
Next idea: Wholesale irrigation store near me gives away damaged pipe free to whoever wants it. It may have been run over, bent or whatever. For the amount of good pipe you would use, (a foot or so) you might ask at places like Ewing, Horizon, Water Savers, John Deere or who ever is near you.
Yes, you then have to buy the fittings. However, they will not leak, and you can include something that will adapt to the tubing from the pump quite easily. Stock item, not DIY.