They shouldn't be allowed the chance ideally. If they weren't fixed they would certainly try and maybe even accomplish it but if the smaller dog is the female it may kill her if she isn't spayed and the pregnancy terminated soon enough or if she made it to whelp she would definitely need a c-section or die. Being mixed breeds, there is no way to try and improve on a breed and the pups produced are likely to end up in the shelter (even pure breds when responsible practices aren't followed) with millions of others that die every year. They are healthier and live longer when fixed. An unspayed female has approximately 40% the life span and much larger vet bills as a direct result of not being spayed and breeding. Please don't allow even a chance for them to breed. Good Luck to you.
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