All scholars agree about the legality of such prayers:

(O Messenger!) When you are among the believers (who are on an expedition and fear that the unbelievers might harm them) and rise to lead the prayer for them, let a party of them stand in prayer with you and retain their arms (while letting the other party take their positions against the enemy). When the first party has prostrated (and finished the rak'at), let them go behind you (to take their positions against the enemy), and let the other party, which has not prayed, come forward and pray with you, being fully prepared against danger and retaining their arms. Those who disbelieve wish that you should be heedless of your weapons and your equipment, so that they might swoop upon you in a surprise attack. But there shall be no blame on you if you lay aside your arms (during prayer) if you are troubled by rain (and the ground impedes your movement), or if you are ill. However, (always) be fully prepared against danger. Surely God has prepared for the unbelievers a shameful, humiliating chastisement. (4:102)