The only possible explanation is that Saints in heaven are not subject to the same time constraints as we here on earth are. Our concept of time is serial - one thing happens after the next, but heaven operates out of time. Jesus said “My Kingdom is not of this world”, so it makes sense that it is outside of time. Lumen Gentium says “They do not cease to intercede with the Father for us, as they proffer the merits which they acquired on earth through the one mediator between God and men, Christ Jesus”. They have all the “time” in heaven to intercede for us, even if we pray all at once.