Is it legal to sign your spouse's name to any contract when she is not present, or withouy power of attorney in the state of PA

You were misinformed. You do not have authority to sign your wife's name to a contract simply becuase she is your spouse. You should demand that the contract be recinded if she is unwilling to sign the contract now. You may remain liabile for contract, but the alarm company may not want only one name and it would then be appropriate for service to be discontinued. If you now have an unpaid balance and are being sued for balance, your defense may not be considered credible, unless the alarm company acknowledges their inappropriate practice.

This is not formal legal advice under an attorney client relationship. I would have to conduct further research to confirm my opinion, but I would put it to the company to provide a citation to the authority they believe supports such pratice.