Is it safe to take this Tamsulosin 0.4mg Tablet PR when I have a 0.5mm stone in both kidneys?

Tamsulosin is actually used quite often to help kidney stones pass more easily, since it relaxes the muscles in the ureter and can ease the discomfort of passing smaller stones. A 0.5mm stone is quite small, so this is a reasonable option in many cases, but tamsulosin can lower blood pressure a little and isn't right for everyone. Since you have stones in both kidneys, please have your urologist confirm the dose and duration that's appropriate for your specific situation before starting.