use client_action_multi

https://doc.openerp.com/6.0/developer/2_4_module_development/action_creation/

For example: Refer sale/wizard/sale_line_invoice.xml

        <record id="action_view_sale_order_line_make_invoice" model="ir.actions.act_window">
            <field name="name">Make Invoices</field>
            <field name="type">ir.actions.act_window</field>
            <field name="res_model">sale.order.line.make.invoice</field>
            <field name="view_type">form</field>
            <field name="view_mode">form</field>
            <field name="view_id" ref="view_sale_order_line_make_invoice"/>
            <field name="target">new</field>
        </record>

        <record model="ir.values" id="sale_order_line_make_invoice">
            <field name="model_id" ref="sale.model_sale_order_line" />
            <field name="name">Make Invoices</field>
            <field name="key2">client_action_multi</field>
            <field name="value" eval="'ir.actions.act_window,' +
                         str(ref('action_view_sale_order_line_make_invoice'))" />
            <field name="key">action</field>
            <field name="model">sale.order.line</field>
        </record>