I'm sure i answered this last week but perhaps i forgot to press Send. if you plant the second row it will take all the light from the first row so it will not flourish. Also planting large bushes is often less effective than smaller ones in the long run. because they take longer to establish. Please could you say why you want a particularly thick hedge - for privacy or to deter any children etc coming through it etc? Different varieties are best for different purposes. if I wanted a thick prickly evergreen hedge i would have gone for one of the larger berberis, which are thick, evergreen, have attractive yellow flowers in spring and can be cut with shears or a hedge trimmer when required (though that would reduce the noimber if flowers).

Generally speaking to encourage a hedge to grow thickly you need well dug fertile soil, keeping the young shrubs watered through the first season and not being in a hurry - trimming the tops as they grow encourages growth to appear lower down and avoids bare trunks appearing at the base. A laurel hedge is best trimmed with secateurs to avoid cutting leaves in half which could then go brown and look messy.