
There are more than one type of basket available for trekking poles. You can get a solid basket, which is designed for summer use, and is designed to stop you sinking into soft ground and to stop the force of the downthrust as you place your pole from causing stones to flick up. Winter baskets commonly have cut out sections, as you have described. The winter, or snow, basket is designed to prevent you sinking into snow by spreading the downforce over a larger surface area than just the tip but allowing a degree of movement of wet snow through the basket so that the "teeth" around the lower edge of the basket to get a sufficient purchase on the harder underlying snow. The idea is similar to that used for snowshoes, which are generally not solid plates, but have holes.
To clip two poles together, side by side, you should use the specifically designed attachment that looks similar to a figure 8 with a gap on each side designed to accomodate the pole shaft. You can also get a special attachment for Leki poles for toasting marshmallows.