Do you think it’s out of order that my boss let two people use the garage I work in on the weekend

Setting aside the futility of rhetorically questioning strangers of no consequence to your livelihood, are you personally invested in or held responsible for the safety of the aforementioned equipment? Is it insured?
Your boss is likely aware of the risk inherent to granting anyone unsupervised access to the premises (weighed according to trust), though he might not deem it sufficiently consequential compared to the financial or social gain. Regardless, whither you stand influence is to be gained through voicing concern over silent consent. Since he's your boss, your capacity for hailing is limited - gripes always go up the chain of command, to his supervising agents if there are any. Or pick an alternative approach, such as insurance, a vault, taking valuables with you (though I imagine you refer to the heavy duty tools). Dissolving the partnership, if willing to consider this option.