The size of these bubbles is growing nearly by the speed of light. So if the boundaries of two such bubbles are close enough to each other, the exponential expansion of the parent space in between them, even if this expansion exists, is negligible relatively to the shrinking distance between the bubbles due to their growth. That's why the bubbles collide after a finite time.
Of course, if the bubbles are far enough, e.g. (or i.e.) behind the horizons of each other, their growth isn't enough to overcome the exponentially growing separation and they cannot collide.
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