An effective requirements engineering (RE) approach must harmonise the need to achieve separation of concerns with the need to satisfy broadly scoped requirements and constraints. Techniques such as use cases and viewpoints help achieve separation of stakeholders' concerns but ensuring their consistency with global requirements and constraints is largely unsupported. In this paper we propose an approach to modularise and compose such crosscutting, aspectual requirements. The approach is based on separating the specification of aspectual requirements, non-aspectual requirements and composition rules in modules representing coherent abstractions and following welldefined templates. The composition rules employ informal, and often concern-specific, actions and operators to specify how an aspectual requirement influences or constrains the behaviour of a set of non-aspectual requirements. We argue that such modularisation makes it possible to establish early trade-offs between aspectual requirements hence providing support for negotiation and subsequent decision-making among stakeholders. At the same time early separation of crosscutting requirements facilitates determination of their mapping and influence on artefacts at later development stages. A realisation of the proposed approach, based on viewpoints and the eXtensible Markup Language (XML), supported by a tool called ARCaDe and a case study of a toll collection system is presented.